From: Eric Niebler <eniebler@fb.com>
Date: Wed, 7 Dec 2016 20:43:21 +0000 (-0800)
Subject: add folly::FixedString, a constexpr-usable string with a fixed-size internal buffer
X-Git-Tag: v2016.12.12.00~12
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=77d5e54744c2ab78ffc06e26085bf18725ec73e1;p=folly.git

add folly::FixedString, a constexpr-usable string with a fixed-size internal buffer

Summary: Inspired by http://wg21.link/p0259, folly::BasicFixedString is a std::string replacement that is usable in constexpr contexts. It uses an internal fixed-size buffer to store up to N characters, where N is a template parameter.

Reviewed By: luciang

Differential Revision: D4249529

fbshipit-source-id: 10f12ea2510a8d21ec60a07a8f15ecc3e35c5431
---

diff --git a/folly/FixedString.h b/folly/FixedString.h
new file mode 100644
index 00000000..0f00e6ed
--- /dev/null
+++ b/folly/FixedString.h
@@ -0,0 +1,2940 @@
+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// @author: Eric Niebler (eniebler)
+// Fixed-size string type, for constexpr string handling.
+
+#pragma once
+
+#include <cassert>
+#include <cstddef>
+#include <initializer_list>
+#include <iosfwd>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include <folly/portability/BitsFunctexcept.h>
+#include <folly/portability/Constexpr.h>
+
+// Define FOLLY_USE_CPP14_CONSTEXPR to be true if the compiler's C++14
+// constexpr support is "good enough".
+#ifndef FOLLY_USE_CPP14_CONSTEXPR
+#if defined(__clang__)
+#define FOLLY_USE_CPP14_CONSTEXPR __cplusplus >= 201300L
+#elif defined(__GNUC__)
+#define FOLLY_USE_CPP14_CONSTEXPR __cplusplus >= 201304L
+#else
+#define FOLLY_USE_CPP14_CONSTEXPR 0 // MSVC?
+#endif
+#endif
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+#define FOLLY_CPP14_CONSTEXPR constexpr
+#else
+#define FOLLY_CPP14_CONSTEXPR inline
+#endif
+
+namespace folly {
+
+template <class Char, std::size_t N>
+class BasicFixedString;
+
+template <std::size_t N>
+using FixedString = BasicFixedString<char, N>;
+
+namespace detail {
+namespace fixedstring {
+
+// This is a template so that the class static npos can be defined in the
+// header.
+template <class = void>
+struct FixedStringBase_ {
+  static constexpr std::size_t npos = static_cast<std::size_t>(-1);
+};
+
+template <class Void>
+constexpr std::size_t FixedStringBase_<Void>::npos;
+
+using FixedStringBase = FixedStringBase_<>;
+
+template <class Char, std::size_t N>
+constexpr std::size_t size(const Char (&)[N]) noexcept {
+  return N - 1u;
+}
+
+template <class Char, std::size_t N>
+constexpr std::size_t size(const BasicFixedString<Char, N>& s) noexcept {
+  return s.size();
+}
+
+// Intentionally NOT constexpr. By making this not constexpr, we make
+// checkOverflow below ill-formed in a constexpr context when the condition
+// it's testing for fails. In this way, precondition violations are reported
+// at compile-time instead of at runtime.
+[[noreturn]] inline void assertOutOfBounds() {
+  assert(false && "Array index out of bounds in BasicFixedString");
+  std::__throw_out_of_range("Array index out of bounds in BasicFixedString");
+}
+
+constexpr std::size_t checkOverflow(std::size_t i, std::size_t max) {
+  return i <= max ? i : (assertOutOfBounds(), max);
+}
+
+constexpr std::size_t checkOverflowOrNpos(std::size_t i, std::size_t max) {
+  return i == FixedStringBase::npos
+      ? max
+      : (i <= max ? i : (assertOutOfBounds(), max));
+}
+
+// Intentionally NOT constexpr. See note above for assertOutOfBounds
+inline void assertOutOfBoundsNothrow() {
+  assert(false && "Array index out of bounds in BasicFixedString");
+}
+
+constexpr std::size_t checkOverflowNothrow(std::size_t i, std::size_t max) {
+  return i <= max ? i : (assertOutOfBoundsNothrow(), i);
+}
+
+// Intentionally NOT constexpr. See note above for assertOutOfBounds
+inline void assertNotNullTerminated() noexcept {
+  assert(
+      false &&
+      "Non-null terminated string used to initialize a BasicFixedString");
+}
+
+// Parsing help for human readers: the following is a constexpr noexcept
+// function that accepts a reference to an array as a parameter and returns
+// a reference to the same array.
+template <class Char, std::size_t N>
+constexpr const Char (&checkNullTerminated(const Char (&a)[N]) noexcept)[N] {
+  // Strange decltype(a)(a) used to make MSVC happy.
+  return a[N - 1u] == Char(0)
+#ifndef NDEBUG
+          // In Debug mode, guard against embedded nulls:
+          && N - 1u == folly::detail::constexpr_strlen_internal(a, 0u)
+#endif
+      ? decltype(a)(a)
+      : (assertNotNullTerminated(), decltype(a)(a));
+}
+
+enum class Cmp : int { LT = -1, EQ = 0, GT = 1 };
+
+template <class Left, class Right>
+constexpr Cmp compare_(
+    const Left& left,
+    std::size_t left_pos,
+    std::size_t left_size,
+    const Right& right,
+    std::size_t right_pos,
+    std::size_t right_size) noexcept {
+  return left_pos == left_size
+      ? (right_pos == right_size ? Cmp::EQ : Cmp::LT)
+      : (right_pos == right_size ? Cmp::GT
+                                 : (left[left_pos] < right[right_pos]
+                                        ? Cmp::LT
+                                        : (left[left_pos] > right[right_pos]
+                                               ? Cmp::GT
+                                               : fixedstring::compare_(
+                                                     left,
+                                                     left_pos + 1u,
+                                                     left_size,
+                                                     right,
+                                                     right_pos + 1u,
+                                                     right_size))));
+}
+
+template <class Left, class Right>
+constexpr bool equal_(
+    const Left& left,
+    std::size_t left_size,
+    const Right& right,
+    std::size_t right_size) noexcept {
+  return left_size == right_size &&
+      Cmp::EQ == compare_(left, 0u, left_size, right, 0u, right_size);
+}
+
+template <class Char, class Left, class Right>
+constexpr Char
+char_at_(const Left& left, const Right& right, std::size_t i) noexcept {
+  return i < fixedstring::size(left)
+      ? left[i]
+      : i < (fixedstring::size(left) + fixedstring::size(right))
+          ? right[i - fixedstring::size(left)]
+          : Char(0);
+}
+
+template <class Char, class Left, class Right>
+constexpr Char char_at_(
+    const Left& left,
+    std::size_t left_pos,
+    std::size_t left_count,
+    const Right& right,
+    std::size_t right_pos,
+    std::size_t right_count,
+    std::size_t i) noexcept {
+  return i < left_pos
+      ? left[i]
+      : (i < right_count + left_pos
+             ? right[i - left_pos + right_pos]
+             : (i < fixedstring::size(left) - left_count + right_count
+                    ? left[i - right_count + left_count]
+                    : Char(0)));
+}
+
+template <class Left, class Right>
+constexpr bool find_at_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return 0u == count || (left[pos + count - 1u] == right[count - 1u] &&
+                         find_at_(left, right, pos, count - 1u));
+}
+
+template <class Char, class Right>
+constexpr bool
+find_one_of_at_(Char ch, const Right& right, std::size_t pos) noexcept {
+  return 0u != pos &&
+      (ch == right[pos - 1u] || find_one_of_at_(ch, right, pos - 1u));
+}
+
+template <class Left, class Right>
+constexpr std::size_t find_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return find_at_(left, right, pos, count)
+      ? pos
+      : fixedstring::size(left) <= pos + count
+          ? FixedStringBase::npos
+          : find_(left, right, pos + 1u, count);
+}
+
+template <class Left, class Right>
+constexpr std::size_t rfind_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return find_at_(left, right, pos, count)
+      ? pos
+      : 0u == pos ? FixedStringBase::npos
+                  : rfind_(left, right, pos - 1u, count);
+}
+
+template <class Left, class Right>
+constexpr std::size_t find_first_of_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return find_one_of_at_(left[pos], right, count)
+      ? pos
+      : fixedstring::size(left) <= pos + 1u
+          ? FixedStringBase::npos
+          : find_first_of_(left, right, pos + 1u, count);
+}
+
+template <class Left, class Right>
+constexpr std::size_t find_first_not_of_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return !find_one_of_at_(left[pos], right, count)
+      ? pos
+      : fixedstring::size(left) <= pos + 1u
+          ? FixedStringBase::npos
+          : find_first_not_of_(left, right, pos + 1u, count);
+}
+
+template <class Left, class Right>
+constexpr std::size_t find_last_of_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return find_one_of_at_(left[pos], right, count)
+      ? pos
+      : 0u == pos ? FixedStringBase::npos
+                  : find_last_of_(left, right, pos - 1u, count);
+}
+
+template <class Left, class Right>
+constexpr std::size_t find_last_not_of_(
+    const Left& left,
+    const Right& right,
+    std::size_t pos,
+    std::size_t count) noexcept {
+  return !find_one_of_at_(left[pos], right, count)
+      ? pos
+      : 0u == pos ? FixedStringBase::npos
+                  : find_last_not_of_(left, right, pos - 1u, count);
+}
+
+struct Helper {
+  template <class Char, class Left, class Right, std::size_t... Is>
+  static constexpr BasicFixedString<Char, sizeof...(Is)> concat_(
+      const Left& left,
+      const Right& right,
+      std::index_sequence<Is...> is) noexcept {
+    return {left, right, is};
+  }
+
+  template <class Char, class Left, class Right, std::size_t... Is>
+  static constexpr BasicFixedString<Char, sizeof...(Is)> replace_(
+      const Left& left,
+      std::size_t left_pos,
+      std::size_t left_count,
+      const Right& right,
+      std::size_t right_pos,
+      std::size_t right_count,
+      std::index_sequence<Is...> is) noexcept {
+    return {left, left_pos, left_count, right, right_pos, right_count, is};
+  }
+};
+
+template <class T>
+FOLLY_CPP14_CONSTEXPR void constexpr_swap(T& a, T& b) noexcept(
+    noexcept(a = T(std::move(a)))) {
+  T tmp((std::move(a)));
+  a = std::move(b);
+  b = std::move(tmp);
+}
+
+// FUTURE: use const_log2 to fold instantiations of BasicFixedString together.
+// All BasicFixedString<C, N> instantiations could share the implementation
+// of BasicFixedString<C, M>, where M is the next highest power of 2 after N.
+//
+// Also, because of alignment of the data_ and size_ members, N should never be
+// smaller than `(alignof(std::size_t)/sizeof(C))-1` (-1 because of the null
+// terminator). OR, create a specialization for BasicFixedString<C, 0u> that
+// does not have a size_ member, since it is unnecessary.
+constexpr std::size_t const_log2(std::size_t N, std::size_t log2 = 0u) {
+  return N / 2u == 0u ? log2 : const_log2(N / 2u, log2 + 1u);
+}
+
+// For constexpr reverse iteration over a BasicFixedString
+template <class T>
+struct ReverseIterator {
+ private:
+  T* p_ = nullptr;
+  struct dummy_ {
+    T* p_ = nullptr;
+  };
+  using other = typename std::conditional<
+      std::is_const<T>::value,
+      ReverseIterator<typename std::remove_const<T>::type>,
+      dummy_>::type;
+
+ public:
+  using value_type = typename std::remove_const<T>::type;
+  using reference = T&;
+  using pointer = T*;
+  using difference_type = std::ptrdiff_t;
+  using iterator_category = std::random_access_iterator_tag;
+
+  constexpr ReverseIterator() = default;
+  constexpr ReverseIterator(const ReverseIterator&) = default;
+  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator=(const ReverseIterator&) =
+      default;
+  constexpr explicit ReverseIterator(T* p) noexcept : p_(p) {}
+  constexpr /* implicit */ ReverseIterator(const other& that) noexcept
+      : p_(that.p_) {}
+  friend constexpr bool operator==(
+      ReverseIterator a,
+      ReverseIterator b) noexcept {
+    return a.p_ == b.p_;
+  }
+  friend constexpr bool operator!=(
+      ReverseIterator a,
+      ReverseIterator b) noexcept {
+    return !(a == b);
+  }
+  constexpr reference operator*() const {
+    return *(p_ - 1);
+  }
+  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator++() noexcept {
+    --p_;
+    return *this;
+  }
+  FOLLY_CPP14_CONSTEXPR ReverseIterator operator++(int)noexcept {
+    auto tmp(*this);
+    --p_;
+    return tmp;
+  }
+  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator--() noexcept {
+    ++p_;
+    return *this;
+  }
+  FOLLY_CPP14_CONSTEXPR ReverseIterator operator--(int)noexcept {
+    auto tmp(*this);
+    ++p_;
+    return tmp;
+  }
+  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator+=(std::ptrdiff_t i) noexcept {
+    p_ -= i;
+    return *this;
+  }
+  friend constexpr ReverseIterator operator+(
+      std::ptrdiff_t i,
+      ReverseIterator that) noexcept {
+    return ReverseIterator{that.p_ - i};
+  }
+  friend constexpr ReverseIterator operator+(
+      ReverseIterator that,
+      std::ptrdiff_t i) noexcept {
+    return ReverseIterator{that.p_ - i};
+  }
+  FOLLY_CPP14_CONSTEXPR ReverseIterator& operator-=(std::ptrdiff_t i) noexcept {
+    p_ += i;
+    return *this;
+  }
+  friend constexpr ReverseIterator operator-(
+      ReverseIterator that,
+      std::ptrdiff_t i) noexcept {
+    return ReverseIterator{that.p_ + i};
+  }
+  friend constexpr std::ptrdiff_t operator-(
+      ReverseIterator a,
+      ReverseIterator b) noexcept {
+    return b.p_ - a.p_;
+  }
+  constexpr reference operator[](std::ptrdiff_t i) const noexcept {
+    return *(*this + i);
+  }
+};
+
+} // namespace fixedstring
+} // namespace detail
+
+// Defined in folly/Range.h
+template <class Iter>
+class Range;
+
+// Defined in folly/Hash.h
+std::uint32_t hsieh_hash32_buf(const void* buf, std::size_t len);
+
+/** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** *
+ * \class BasicFixedString
+ *
+ * \tparam Char The character type. Must be a scalar type.
+ * \tparam N The capacity and max size of string instances of this type.
+ *
+ * \brief A class for holding up to `N` characters of type `Char` that is
+ *        amenable to `constexpr` string manipulation. It is guaranteed to not
+ *        perform any dynamic allocation.
+ *
+ * `BasicFixedString` is a `std::string` work-alike that stores characters in an
+ * internal buffer. It has minor interface differences that make it easy to work
+ * with strings in a `constexpr` context.
+ *
+ * \par Example:
+ * \par
+ * \code
+ * constexpr auto hello = makeFixedString("hello");         // a FixedString<5>
+ * constexpr auto world = makeFixedString("world");         // a FixedString<5>
+ * constexpr auto hello_world = hello + ' ' + world + '!';  // a FixedString<12>
+ * static_assert(hello_world == "hello world!", "neato!");
+ * \endcode
+ * \par
+ * `FixedString<N>` is an alias for `BasicFixedString<char, N>`.
+ *
+ * \par Constexpr and In-place Mutation
+ * \par
+ * On a C++14 compiler, `BasicFixedString` supports the full `std::string`
+ * interface as `constexpr` member functions. On a C++11 compiler, the mutating
+ * members are not `constexpr`, but non-mutating alternatives, which create a
+ * new string, can be used instead. For example, instead of this:
+ * \par
+ * \code
+ * constexpr FixedString<10> replace_example_cpp14() {
+ *   FixedString<10> test{"****"};
+ *   test.replace(1, 2, "!!!!");
+ *   return test; // returns "*!!!!*"
+ * }
+ * \endcode
+ * \par
+ * You might write this instead:
+ * \par
+ * \code
+ * constexpr FixedString<10> replace_example_cpp11() {
+ *   // GNU compilers have an extension that make it possible to create
+ *   // FixedString objects with a `""_fs` user-defined literal.
+ *   using namespace folly;
+ *   return makeFixedString("****").creplace(1, 2, "!!!!"); // "*!!!!*"
+ * }
+ * \endcode
+ *
+ * \par User-defined Literals
+ * Instead of using the `folly::makeFixedString` helper function, you can use
+ * a user-defined literal to make `FixedString` instances. The UDL feature of
+ * C++ has some limitations that make this less than ideal; you must tell the
+ * compiler roughly how many characters are in the string. The suffixes `_fs4`,
+ * `_fs8`, `_fs16`, `_fs32`, `_fs64`, and `_fs128` exist to create instances
+ * of types `FixedString<4>`, `FixedString<8>`, etc. For example:
+ * \par
+ * \code
+ * using namespace folly::StringLiterals;
+ * constexpr auto hello = "hello"_fs8; // A FixedString<8> containing "hello"
+ * \endcode
+ * \par
+ * See Error Handling below for what to expect when you try to exceed the
+ * capacity of a `FixedString` by storing too many characters in it.
+ * \par
+ * If your compiler supports GNU extensions, there is one additional suffix you
+ * can use: `_fs`. This suffix always creates `FixedString` objects of exactly
+ * the right size. For example:
+ * \par
+ * \code
+ * using namespace folly::StringLiterals;
+ * // NOTE: Only works on compilers with GNU extensions enabled. Clang and
+ * // gcc support this (-Wgnu-string-literal-operator-template):
+ * constexpr auto hello = "hello"_fs; // A FixedString<5> containing "hello"
+ * \endcode
+ *
+ * \par Error Handling:
+ * The capacity of a `BasicFixedString` is set at compile time. When the user
+ * asks the string to exceed its capacity, one of three things will happen,
+ * depending on the context:
+ *\par
+ *  -# If the attempt is made while evaluating a constant expression, the
+ *     program will fail to compile.
+ *  -# Otherwise, if the program is being run in debug mode, it will `assert`.
+ *  -# Otherwise, the failed operation will throw a `std::out_of_range`
+ *     exception.
+ *\par
+ * This is also the case if an invalid offset is passed to any member function,
+ * or if `pop_back` or `cpop_back` is called on an empty `BasicFixedString`.
+ *
+ * Member functions documented as having preconditions will assert in Debug
+ * mode (`!defined(NDEBUG)`) on precondition failures. Those documented with
+ * \b Throws clauses will throw the specified exception on failure. Those with
+ * both a precondition and a \b Throws clause will assert in Debug and throw
+ * in Release mode.
+ */
+template <class Char, std::size_t N>
+class BasicFixedString : private detail::fixedstring::FixedStringBase {
+ private:
+  template <class, std::size_t>
+  friend class BasicFixedString;
+  friend struct detail::fixedstring::Helper;
+
+  Char data_[N + 1u]; // +1 for the null terminator
+  std::size_t size_; // Nbr of chars, not incl. null terminator. size_ <= N.
+
+  using Indices = std::make_index_sequence<N>;
+
+  template <class That, std::size_t... Is>
+  constexpr BasicFixedString(
+      const That& that,
+      std::size_t size,
+      std::index_sequence<Is...>,
+      std::size_t pos = 0,
+      std::size_t count = npos) noexcept
+      : data_{(Is < (size - pos) && Is < count ? that[Is + pos] : Char(0))...,
+              Char(0)},
+        size_{folly::constexpr_min(size - pos, count)} {}
+
+  template <std::size_t... Is>
+  constexpr BasicFixedString(
+      std::size_t count,
+      Char ch,
+      std::index_sequence<Is...>) noexcept
+      : data_{((Is < count) ? ch : Char(0))..., Char(0)}, size_{count} {}
+
+  // Concatenation constructor
+  template <class Left, class Right, std::size_t... Is>
+  constexpr BasicFixedString(
+      const Left& left,
+      const Right& right,
+      std::index_sequence<Is...>) noexcept
+      : data_{detail::fixedstring::char_at_<Char>(left, right, Is)..., Char(0)},
+        size_{detail::fixedstring::size(left) +
+              detail::fixedstring::size(right)} {}
+
+  // Replace constructor
+  template <class Left, class Right, std::size_t... Is>
+  constexpr BasicFixedString(
+      const Left& left,
+      std::size_t left_pos,
+      std::size_t left_count,
+      const Right& right,
+      std::size_t right_pos,
+      std::size_t right_count,
+      std::index_sequence<Is...>) noexcept
+      : data_{detail::fixedstring::char_at_<Char>(
+                  left,
+                  left_pos,
+                  left_count,
+                  right,
+                  right_pos,
+                  right_count,
+                  Is)...,
+              Char(0)},
+        size_{detail::fixedstring::size(left) - left_count + right_count} {}
+
+ public:
+  using size_type = std::size_t;
+  using difference_type = std::ptrdiff_t;
+  using reference = Char&;
+  using const_reference = const Char&;
+  using pointer = Char*;
+  using const_pointer = const Char*;
+  using iterator = Char*;
+  using const_iterator = const Char*;
+  using reverse_iterator = detail::fixedstring::ReverseIterator<Char>;
+  using const_reverse_iterator =
+      detail::fixedstring::ReverseIterator<const Char>;
+
+  using detail::fixedstring::FixedStringBase::npos;
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Default construct
+   * \post `size() == 0`
+   * \post `at(0) == Char(0)`
+   */
+  constexpr BasicFixedString() : data_{}, size_{} {}
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Copy construct
+   * \post `size() == that.size()`
+   * \post `0 == strncmp(data(), that.data(), size())`
+   * \post `at(size()) == Char(0)`
+   */
+  constexpr BasicFixedString(const BasicFixedString& /*that*/) = default;
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct from a differently-sized BasicFixedString
+   * \pre `that.size() <= N`
+   * \post `size() == that.size()`
+   * \post `0 == strncmp(data(), that.data(), size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when that.size() > N. When M <= N, this
+   *   constructor will never throw.
+   * \note Conversions from larger-capacity BasicFixedString objects to smaller
+   *   ones (`M > N`) are allowed as long as the *size()* of the source string
+   *   is small enough.
+   */
+  template <std::size_t M>
+  constexpr /* implicit */ BasicFixedString(
+      const BasicFixedString<Char, M>& that) noexcept(M <= N)
+      : BasicFixedString{that, 0u, that.size_} {}
+
+  // Why is this deleted? To avoid confusion with the constructor that takes
+  // a const Char* and a count.
+  template <std::size_t M>
+  constexpr BasicFixedString(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) noexcept(false) = delete;
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct from an BasicFixedString, an offset, and a count
+   * \param that The source string
+   * \param pos The starting position in `that`
+   * \param count The number of characters to copy. If `npos`, `count` is taken
+   *              to be `that.size()-pos`.
+   * \pre `pos <= that.size()`
+   * \pre `count <= that.size()-pos && count <= N`
+   * \post `size() == count`
+   * \post `0 == strncmp(data(), that.data()+pos, size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when pos+count > that.size(), or when
+   *        `count > N`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos,
+      std::size_t count) noexcept(false)
+      : BasicFixedString{
+            that.data_,
+            that.size_,
+            std::make_index_sequence<(M < N ? M : N)>{},
+            pos,
+            detail::fixedstring::checkOverflow(
+                detail::fixedstring::checkOverflowOrNpos(
+                    count,
+                    that.size_ -
+                        detail::fixedstring::checkOverflow(pos, that.size_)),
+                N)} {}
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct from a string literal
+   * \pre `M-1 <= N`
+   * \pre `that[M-1] == Char(0)`
+   * \post `0 == strncmp(data(), that, M-1)`
+   * \post `size() == M-1`
+   * \post `at(size()) == Char(0)`
+   */
+  template <std::size_t M, class = typename std::enable_if<(M - 1u <= N)>::type>
+  constexpr /* implicit */ BasicFixedString(const Char (&that)[M]) noexcept
+      : BasicFixedString{detail::fixedstring::checkNullTerminated(that),
+                         M - 1u,
+                         std::make_index_sequence<M - 1u>{}} {}
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct from a `const Char*` and count
+   * \pre `that` points to an array of at least `count` characters.
+   * \pre `count <= N`
+   * \post `size() == count`
+   * \post `0 == strncmp(data(), that, size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when count > N
+   */
+  constexpr BasicFixedString(const Char* that, std::size_t count) noexcept(
+      false)
+      : BasicFixedString{that,
+                         detail::fixedstring::checkOverflow(count, N),
+                         Indices{}} {}
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct an BasicFixedString that contains `count` characters, all
+   *   of which are `ch`.
+   * \pre `count <= N`
+   * \post `size() == count`
+   * \post `npos == find_first_not_of(ch)`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when count > N
+   */
+  constexpr BasicFixedString(std::size_t count, Char ch) noexcept(false)
+      : BasicFixedString{detail::fixedstring::checkOverflow(count, N),
+                         ch,
+                         Indices{}} {}
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct an BasicFixedString from a `std::initializer_list` of
+   *   characters.
+   * \pre `il.size() <= N`
+   * \post `size() == count`
+   * \post `0 == strncmp(data(), il.begin(), size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when il.size() > N
+   */
+  constexpr BasicFixedString(std::initializer_list<Char> il) noexcept(false)
+      : BasicFixedString{il.begin(), il.size()} {}
+
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+      const BasicFixedString&) noexcept = default;
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assign from a `BasicFixedString<Char, M>`.
+   * \pre `that.size() <= N`
+   * \post `size() == that.size()`
+   * \post `0 == strncmp(data(), that.begin(), size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when that.size() > N. When M <= N, this
+   *   assignment operator will never throw.
+   * \note Assignments from larger-capacity BasicFixedString objects to smaller
+   *   ones (`M > N`) are allowed as long as the *size* of the source string is
+   *   small enough.
+   * \return `*this`
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+      const BasicFixedString<Char, M>& that) noexcept(M <= N) {
+    detail::fixedstring::checkOverflow(that.size_, N);
+    size_ = that.copy(data_, that.size_);
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assign from a null-terminated array of characters.
+   * \pre `M < N`
+   * \pre `that` has no embedded null characters
+   * \pre `that[M-1]==Char(0)`
+   * \post `size() == M-1`
+   * \post `0 == strncmp(data(), that, size())`
+   * \post `at(size()) == Char(0)`
+   * \return `*this`
+   */
+  template <std::size_t M, class = typename std::enable_if<(M - 1u <= N)>::type>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+      const Char (&that)[M]) noexcept {
+    return assign(detail::fixedstring::checkNullTerminated(that), M - 1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assign from an `initializer_list` of characters.
+   * \pre `il.size() <= N`
+   * \post `size() == il.size()`
+   * \post `0 == strncmp(data(), il.begin(), size())`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when il.size() > N
+   * \return `*this`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator=(
+      std::initializer_list<Char> il) noexcept(false) {
+    detail::fixedstring::checkOverflow(il.size(), N);
+    for (std::size_t i = 0u; i < il.size(); ++i) {
+      data_[i] = il.begin()[i];
+    }
+    size_ = il.size();
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Conversion to folly::Range
+   * \return `Range<Iter>{begin(), end()}`
+   */
+  template <
+      class Iter,
+      class = typename std::enable_if<
+          std::is_convertible<Char*, Iter>::value>::type>
+  FOLLY_CPP14_CONSTEXPR /* implicit */ operator Range<Iter>() noexcept {
+    return {begin(), end()};
+  }
+
+  /**
+   * \overload
+   */
+  template <
+      class Iter,
+      class = typename std::enable_if<
+          std::is_convertible<const Char*, Iter>::value>::type>
+  constexpr /* implicit */ operator Range<Iter>() const noexcept {
+    return {begin(), end()};
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Conversion to folly::Range
+   * \return `Range<Char*>{begin(), end()}`
+   */
+  FOLLY_CPP14_CONSTEXPR Range<Char*> toRange() noexcept {
+    return {begin(), end()};
+  }
+
+  /**
+   * \overload
+   */
+  constexpr Range<const Char*> toRange() const noexcept {
+    return {begin(), end()};
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Conversion to std::basic_string<Char>
+   * \return `std::basic_string<Char>{begin(), end()}`
+   */
+  /* implicit */ operator std::basic_string<Char>() const noexcept(false) {
+    return std::basic_string<Char>{begin(), end()};
+  }
+
+  std::basic_string<Char> toStdString() const noexcept(false) {
+    return std::basic_string<Char>{begin(), end()};
+  }
+
+  // Think hard about whether this is a good idea. It's certainly better than
+  // an implicit conversion to `const Char*` since `delete "hi"_fs` will fail
+  // to compile. But it creates ambiguities when passing a FixedString to an
+  // API that has overloads for `const char*` and `folly::Range`, for instance.
+  // using ArrayType = Char[N];
+  // FOLLY_CPP14_CONSTEXPR /* implicit */ operator ArrayType&() noexcept {
+  //   return data_;
+  // }
+
+  // using ConstArrayType = const Char[N];
+  // constexpr /* implicit */ operator ConstArrayType&() const noexcept {
+  //   return data_;
+  // }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assigns a sequence of `count` characters of value `ch`.
+   * \param count The count of characters.
+   * \param ch
+   * \pre `count <= N`
+   * \post `size() == count`
+   * \post `npos == find_first_not_of(ch)`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when count > N
+   * \return `*this`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+      std::size_t count,
+      Char ch) noexcept(false) {
+    detail::fixedstring::checkOverflow(count, N);
+    for (std::size_t i = 0u; i < count; ++i) {
+      data_[i] = ch;
+    }
+    size_ = count;
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assigns characters from an `BasicFixedString` to this object.
+   * \note Equivalent to `assign(that, 0, that.size())`
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+      const BasicFixedString<Char, M>& that) noexcept(M <= N) {
+    return *this = that;
+  }
+
+  // Why is this overload deleted? So users aren't confused by the difference
+  // between str.assign("foo", N) and str.assign("foo"_fs, N). In the former,
+  // N is a count of characters. In the latter, it would be a position, which
+  // totally changes the meaning of the code.
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) noexcept(false) = delete;
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assigns `count` characters from an `BasicFixedString` to this object,
+   *   starting at position `pos` in the source object.
+   * \param that The source string.
+   * \param pos The starting position in the source string.
+   * \param count The number of characters to copy. If `npos`, `count` is taken
+   *              to be `that.size()-pos`.
+   * \pre `pos <= that.size()`
+   * \pre `count <= that.size()-pos`
+   * \pre `count <= N`
+   * \post `size() == count`
+   * \post `0 == strncmp(data(), that.begin() + pos, count)`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when pos > that.size() or count > that.size()-pos
+   *        or count > N.
+   * \return `*this`
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos,
+      std::size_t count) noexcept(false) {
+    detail::fixedstring::checkOverflow(pos, that.size_);
+    return assign(
+        that.data_ + pos,
+        detail::fixedstring::checkOverflowOrNpos(count, that.size_ - pos));
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assigns characters from an `BasicFixedString` to this object.
+   * \pre `that` contains no embedded nulls.
+   * \pre `that[M-1] == Char(0)`
+   * \note Equivalent to `assign(that, M - 1)`
+   */
+  template <std::size_t M, class = typename std::enable_if<(M - 1u <= N)>::type>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+      const Char (&that)[M]) noexcept {
+    return assign(detail::fixedstring::checkNullTerminated(that), M - 1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Assigns `count` characters from a range of characters to this object.
+   * \param that A pointer to a range of characters.
+   * \param count The number of characters to copy.
+   * \pre `that` points to at least `count` characters.
+   * \pre `count <= N`
+   * \post `size() == count`
+   * \post `0 == strncmp(data(), that, count)`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range when count > N
+   * \return `*this`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& assign(
+      const Char* that,
+      std::size_t count) noexcept(false) {
+    detail::fixedstring::checkOverflow(count, N);
+    for (std::size_t i = 0u; i < count; ++i) {
+      data_[i] = that[i];
+    }
+    size_ = count;
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Swap the contents of this string with `that`.
+   */
+  FOLLY_CPP14_CONSTEXPR void swap(BasicFixedString& that) noexcept {
+    // less-than-or-equal here to copy the null terminator:
+    for (std::size_t i = 0u; i <= folly::constexpr_max(size_, that.size_);
+         ++i) {
+      detail::fixedstring::constexpr_swap(data_[i], that.data_[i]);
+    }
+    detail::fixedstring::constexpr_swap(size_, that.size_);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Return a pointer to a range of `size()+1` characters, the last of which
+   * is `Char(0)`.
+   */
+  FOLLY_CPP14_CONSTEXPR Char* data() noexcept {
+    return data_;
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char* data() const noexcept {
+    return data_;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * \return `data()`.
+   */
+  constexpr const Char* c_str() const noexcept {
+    return data_;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * \return `data()`.
+   */
+  FOLLY_CPP14_CONSTEXPR Char* begin() noexcept {
+    return data_;
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char* begin() const noexcept {
+    return data_;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * \return `data()`.
+   */
+  constexpr const Char* cbegin() const noexcept {
+    return begin();
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * \return `data() + size()`.
+   */
+  FOLLY_CPP14_CONSTEXPR Char* end() noexcept {
+    return data_ + size_;
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char* end() const noexcept {
+    return data_ + size_;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * \return `data() + size()`.
+   */
+  constexpr const Char* cend() const noexcept {
+    return end();
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Returns a reverse iterator to the first character of the reversed string.
+   * It corresponds to the last + 1 character of the non-reversed string.
+   */
+  FOLLY_CPP14_CONSTEXPR reverse_iterator rbegin() noexcept {
+    return reverse_iterator{data_ + size_};
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const_reverse_iterator rbegin() const noexcept {
+    return const_reverse_iterator{data_ + size_};
+  }
+
+  /**
+   * \note Equivalent to `rbegin()` on a const-qualified reference to `*this`.
+   */
+  constexpr const_reverse_iterator crbegin() const noexcept {
+    return rbegin();
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Returns a reverse iterator to the last + 1 character of the reversed
+   * string. It corresponds to the first character of the non-reversed string.
+   */
+  FOLLY_CPP14_CONSTEXPR reverse_iterator rend() noexcept {
+    return reverse_iterator{data_};
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const_reverse_iterator rend() const noexcept {
+    return const_reverse_iterator{data_};
+  }
+
+  /**
+   * \note Equivalent to `rend()` on a const-qualified reference to `*this`.
+   */
+  constexpr const_reverse_iterator crend() const noexcept {
+    return rend();
+  }
+
+  /**
+   * \return The number of `Char` elements in the string.
+   */
+  constexpr std::size_t size() const noexcept {
+    return size_;
+  }
+
+  /**
+   * \return The number of `Char` elements in the string.
+   */
+  constexpr std::size_t length() const noexcept {
+    return size_;
+  }
+
+  /**
+   * \return True if and only if `size() == 0`.
+   */
+  constexpr bool empty() const noexcept {
+    return 0u == size_;
+  }
+
+  /**
+   * \return `N`.
+   */
+  static constexpr std::size_t capacity() noexcept {
+    return N;
+  }
+
+  /**
+   * \return `N`.
+   */
+  static constexpr std::size_t max_size() noexcept {
+    return N;
+  }
+
+  // We would need to reimplement folly::Hash to make this
+  // constexpr. :-(
+  std::uint32_t hash() const noexcept {
+    return folly::hsieh_hash32_buf(data_, size_);
+  }
+
+  /**
+   * \note `at(size())` is allowed will return `Char(0)`.
+   * \return `*(data() + i)`
+   * \throw std::out_of_range when i > size()
+   */
+  FOLLY_CPP14_CONSTEXPR Char& at(std::size_t i) noexcept(false) {
+    return i <= size_
+        ? data_[i]
+        : (std::__throw_out_of_range("Out of range in BasicFixedString::at"),
+           data_[size_]);
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char& at(std::size_t i) const noexcept(false) {
+    return i <= size_
+        ? data_[i]
+        : (std::__throw_out_of_range("Out of range in BasicFixedString::at"),
+           data_[size_]);
+  }
+
+  /**
+   * \pre `i <= size()`
+   * \note `(*this)[size()]` is allowed will return `Char(0)`.
+   * \return `*(data() + i)`
+   */
+  FOLLY_CPP14_CONSTEXPR Char& operator[](std::size_t i) noexcept {
+#ifdef NDEBUG
+    return data_[i];
+#else
+    return data_[detail::fixedstring::checkOverflowNothrow(i, size_)];
+#endif
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char& operator[](std::size_t i) const noexcept {
+#ifdef NDEBUG
+    return data_[i];
+#else
+    return data_[detail::fixedstring::checkOverflowNothrow(i, size_)];
+#endif
+  }
+
+  /**
+   * \note Equivalent to `(*this)[0]`
+   */
+  FOLLY_CPP14_CONSTEXPR Char& front() noexcept(false) {
+    return (*this)[0u];
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char& front() const noexcept(false) {
+    return (*this)[0u];
+  }
+
+  /**
+   * \note Equivalent to `at(size()-1)`
+   * \pre `!empty()`
+   */
+  FOLLY_CPP14_CONSTEXPR Char& back() noexcept(false) {
+#ifdef NDEBUG
+    return data_[size_ - 1u];
+#else
+    return data_[size_ - detail::fixedstring::checkOverflowNothrow(1u, size_)];
+#endif
+  }
+
+  /**
+   * \overload
+   */
+  constexpr const Char& back() const noexcept(false) {
+#ifdef NDEBUG
+    return data_[size_ - 1u];
+#else
+    return data_[size_ - detail::fixedstring::checkOverflowNothrow(1u, size_)];
+#endif
+  }
+
+  /**
+   * Clears the contents of this string.
+   * \post `size() == 0u`
+   * \post `at(size()) == Char(0)`
+   */
+  FOLLY_CPP14_CONSTEXPR void clear() noexcept {
+    data_[0u] = Char(0);
+    size_ = 0u;
+  }
+
+  /**
+   * \note Equivalent to `append(1u, ch)`.
+   */
+  FOLLY_CPP14_CONSTEXPR void push_back(Char ch) noexcept(false) {
+    detail::fixedstring::checkOverflow(1u, N - size_);
+    data_[size_] = ch;
+    data_[++size_] = Char(0);
+  }
+
+  /**
+   * \note Equivalent to `cappend(1u, ch)`.
+   */
+  constexpr BasicFixedString<Char, N + 1u> cpush_back(Char ch) const noexcept {
+    return cappend(ch);
+  }
+
+  /**
+   * Removes the last character from the string.
+   * \pre `!empty()`
+   * \post `size()` is one fewer than before calling `pop_back()`.
+   * \post `at(size()) == Char(0)`
+   * \post The characters in the half-open range `[0,size()-1)` are unmodified.
+   * \throw std::out_of_range if empty().
+   */
+  FOLLY_CPP14_CONSTEXPR void pop_back() noexcept(false) {
+    detail::fixedstring::checkOverflow(1u, size_);
+    --size_;
+    data_[size_] = Char(0);
+  }
+
+  /**
+   * Returns a new string with the first `size()-1` characters from this string.
+   * \pre `!empty()`
+   * \note Equivalent to `BasicFixedString<Char, N-1u>{*this, 0u, size()-1u}`
+   * \throw std::out_of_range if empty().
+   */
+  constexpr BasicFixedString<Char, N - 1u> cpop_back() const noexcept(false) {
+    return {*this, 0u, size_ - detail::fixedstring::checkOverflow(1u, size_)};
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Appends `count` copies of `ch` to this string.
+   * \pre `count + old_size <= N`
+   * \post The first `old_size` characters of the string are unmodified.
+   * \post `size() == old_size + count`
+   * \throw std::out_of_range if count > N - size().
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+      std::size_t count,
+      Char ch) noexcept(false) {
+    detail::fixedstring::checkOverflow(count, N - size_);
+    for (std::size_t i = 0u; i < count; ++i)
+      data_[size_ + i] = ch;
+    size_ += count;
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /**
+   * \note Equivalent to `append(*this, 0, that.size())`.
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+      const BasicFixedString<Char, M>& that) noexcept(false) {
+    return append(that, 0u, that.size_);
+  }
+
+  // Why is this overload deleted? So as not to get confused with
+  // append("null-terminated", N), where N would be a count instead
+  // of a position.
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) noexcept(false) = delete;
+
+  /**
+   * Appends `count` characters from another string to this one, starting at a
+   * given offset, `pos`.
+   * \param that The source string.
+   * \param pos The starting position in the source string.
+   * \param count The number of characters to append. If `npos`, `count` is
+   *              taken to be `that.size()-pos`.
+   * \pre `pos <= that.size()`
+   * \pre `count <= that.size() - pos`
+   * \pre `old_size + count <= N`
+   * \post The first `old_size` characters of the string are unmodified.
+   * \post `size() == old_size + count`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range if pos + count > that.size() or if
+   *        `old_size + count > N`.
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos,
+      std::size_t count) noexcept(false) {
+    detail::fixedstring::checkOverflow(pos, that.size_);
+    count = detail::fixedstring::checkOverflowOrNpos(count, that.size_ - pos);
+    detail::fixedstring::checkOverflow(count, N - size_);
+    for (std::size_t i = 0u; i < count; ++i)
+      data_[size_ + i] = that[pos + i];
+    size_ += count;
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /**
+   * \note Equivalent to `append(that, strlen(that))`.
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(const Char* that) noexcept(
+      false) {
+    return append(that, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Appends `count` characters from the specified character array.
+   * \pre `that` points to a range of at least `count` characters.
+   * \pre `count + old_size <= N`
+   * \post The first `old_size` characters of the string are unmodified.
+   * \post `size() == old_size + count`
+   * \post `at(size()) == Char(0)`
+   * \throw std::out_of_range if old_size + count > N.
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& append(
+      const Char* that,
+      std::size_t count) noexcept(false) {
+    detail::fixedstring::checkOverflow(count, N - size_);
+    for (std::size_t i = 0u; i < count; ++i)
+      data_[size_ + i] = that[i];
+    size_ += count;
+    data_[size_] = Char(0);
+    return *this;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Creates a new string by appending a character to an existing string, which
+   *   is left unmodified.
+   * \note Equivalent to `*this + ch`
+   */
+  constexpr BasicFixedString<Char, N + 1u> cappend(Char ch) const noexcept {
+    return *this + ch;
+  }
+
+  /**
+   * Creates a new string by appending a string to an existing string, which
+   *   is left unmodified.
+   * \note Equivalent to `*this + ch`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> cappend(
+      const BasicFixedString<Char, M>& that) const noexcept {
+    return *this + that;
+  }
+
+  // Deleted to avoid confusion with append("char*", N), where N is a count
+  // instead of a position.
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> cappend(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) = delete;
+
+  /**
+   * Creates a new string by appending characters from one string to another,
+   *   which is left unmodified.
+   * \note Equivalent to `*this + that.substr(pos, count)`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> cappend(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return creplace(size_, 0u, that, pos, count);
+  }
+
+  /**
+   * Creates a new string by appending a string literal to a string,
+   *   which is left unmodified.
+   * \note Equivalent to `*this + that`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u> cappend(
+      const Char (&that)[M]) const noexcept {
+    return creplace(size_, 0u, that);
+  }
+
+  // Deleted to avoid confusion with append("char*", N), where N is a count
+  // instead of a position
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u> cappend(
+      const Char (&that)[M],
+      std::size_t pos) const noexcept(false) = delete;
+
+  /**
+   * Creates a new string by appending characters from one string to another,
+   *   which is left unmodified.
+   * \note Equivalent to `*this + makeFixedString(that).substr(pos, count)`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u>
+  cappend(const Char (&that)[M], std::size_t pos, std::size_t count) const
+      noexcept(false) {
+    return creplace(size_, 0u, that, pos, count);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Appends characters from a null-terminated string literal to this string.
+   * \note Equivalent to `append(that)`.
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(const Char* that) noexcept(
+      false) {
+    return append(that);
+  }
+
+  /**
+   * Appends characters from another string to this one.
+   * \note Equivalent to `append(that)`.
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(
+      const BasicFixedString<Char, M>& that) noexcept(false) {
+    return append(that, 0u, that.size_);
+  }
+
+  /**
+   * Appends a character to this string.
+   * \note Equivalent to `push_back(ch)`.
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(Char ch) noexcept(false) {
+    push_back(ch);
+    return *this;
+  }
+
+  /**
+   * Appends characters from an `initializer_list` to this string.
+   * \note Equivalent to `append(il.begin(), il.size())`.
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& operator+=(
+      std::initializer_list<Char> il) noexcept(false) {
+    return append(il.begin(), il.size());
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Erase all characters from this string.
+   * \note Equivalent to `clear()`
+   * \return *this;
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& erase() noexcept {
+    clear();
+    return *this;
+  }
+
+  /**
+   * Erases `count` characters from position `pos`. If `count` is `npos`,
+   *   erases from `pos` to the end of the string.
+   * \pre `pos <= size()`
+   * \pre `count <= size() - pos || count == npos`
+   * \post `size() == old_size - min(count, old_size - pos)`
+   * \post `at(size()) == Char(0)`
+   * \return *this;
+   * \throw std::out_of_range when pos > size().
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& erase(
+      std::size_t pos,
+      std::size_t count = npos) noexcept(false) {
+    using A = const Char[1];
+    return replace(
+        pos,
+        detail::fixedstring::checkOverflowOrNpos(
+            count, size_ - detail::fixedstring::checkOverflow(pos, size_)),
+        A{Char(0)},
+        0u);
+  }
+
+  /**
+   * \note Equivalent to `erase(first - data(), 1)`
+   * \return A pointer to the first character after the erased character.
+   */
+  FOLLY_CPP14_CONSTEXPR Char* erase(const Char* first) noexcept(false) {
+    erase(first - data_, 1u);
+    return data_ + (first - data_);
+  }
+
+  /**
+   * \note Equivalent to `erase(first - data(), last - first)`
+   * \return A pointer to the first character after the erased characters.
+   */
+  FOLLY_CPP14_CONSTEXPR Char* erase(
+      const Char* first,
+      const Char* last) noexcept(false) {
+    erase(first - data_, last - first);
+    return data_ + (first - data_);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Create a new string by erasing all the characters from this string.
+   * \note Equivalent to `BasicFixedString<Char, 0>{}`
+   */
+  constexpr BasicFixedString<Char, 0u> cerase() const noexcept {
+    return {};
+  }
+
+  /**
+   * Create a new string by erasing all the characters after position `pos` from
+   *   this string.
+   * \note Equivalent to `creplace(pos, min(count, pos - size()), "")`
+   */
+  constexpr BasicFixedString cerase(std::size_t pos, std::size_t count = npos)
+      const noexcept(false) {
+    using A = const Char[1];
+    return creplace(
+        pos,
+        detail::fixedstring::checkOverflowOrNpos(
+            count, size_ - detail::fixedstring::checkOverflow(pos, size_)),
+        A{Char(0)});
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Compare two strings for lexicographical ordering.
+   * \note Equivalent to
+   * `compare(0, size(), that.data(), that.size())`
+   */
+  template <std::size_t M>
+  constexpr int compare(const BasicFixedString<Char, M>& that) const noexcept {
+    return compare(0u, size_, that, 0u, that.size_);
+  }
+
+  /**
+   * Compare two strings for lexicographical ordering.
+   * \note Equivalent to
+   * `compare(this_pos, this_count, that.data(), that.size())`
+   */
+  template <std::size_t M>
+  constexpr int compare(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const BasicFixedString<Char, M>& that) const noexcept(false) {
+    return compare(this_pos, this_count, that, 0u, that.size_);
+  }
+
+  /**
+   * Compare two strings for lexicographical ordering.
+   * \note Equivalent to
+   * `compare(this_pos, this_count, that.data() + that_pos, that_count)`
+   */
+  template <std::size_t M>
+  constexpr int compare(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos,
+      std::size_t that_count) const noexcept(false) {
+    return static_cast<int>(detail::fixedstring::compare_(
+        *this,
+        detail::fixedstring::checkOverflow(this_pos, size_),
+        detail::fixedstring::checkOverflow(this_count, size_ - this_pos) +
+            this_pos,
+        that,
+        detail::fixedstring::checkOverflow(that_pos, that.size_),
+        detail::fixedstring::checkOverflow(that_count, that.size_ - that_pos) +
+            that_pos));
+  }
+
+  /**
+   * Compare two strings for lexicographical ordering.
+   * \note Equivalent to `compare(0, size(), that, strlen(that))`
+   */
+  constexpr int compare(const Char* that) const noexcept {
+    return compare(0u, size_, that, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Compare two strings for lexicographical ordering.
+   * \note Equivalent to
+   *   `compare(this_pos, this_count, that, strlen(that))`
+   */
+  constexpr int compare(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const Char* that) const noexcept(false) {
+    return compare(this_pos, this_count, that, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Compare two strings for lexicographical ordering.
+   *
+   * Let `A` be the the
+   *   character sequence {`(*this)[this_pos]`, ...
+   *   `(*this)[this_pos + this_count - 1]`}. Let `B` be the character sequence
+   *   {`that[0]`, ...`that[count - 1]`}. Then...
+   *
+   * \return
+   *   - `< 0` if `A` is ordered before the `B`
+   *   - `> 0` if `B` is ordered before `A`
+   *   - `0` if `A` equals `B`.
+   *
+   * \throw std::out_of_range if this_pos + this_count > size().
+   */
+  constexpr int compare(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const Char* that,
+      std::size_t that_count) const noexcept(false) {
+    return static_cast<int>(detail::fixedstring::compare_(
+        *this,
+        detail::fixedstring::checkOverflow(this_pos, size_),
+        detail::fixedstring::checkOverflowOrNpos(this_count, size_ - this_pos) +
+            this_pos,
+        that,
+        0u,
+        that_count));
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Return a substring from `pos` to the end of the string.
+   * \note Equivalent to `BasicFixedString{*this, pos}`
+   */
+  constexpr BasicFixedString substr(std::size_t pos) const noexcept(false) {
+    return {*this, pos};
+  }
+
+  /**
+   * Return a substring from `pos` to the end of the string.
+   * \note Equivalent to `BasicFixedString{*this, pos, count}`
+   */
+  constexpr BasicFixedString substr(std::size_t pos, std::size_t count) const
+      noexcept(false) {
+    return {*this, pos, count};
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Replace the characters in the range denoted by the half-open range
+   *   [`first`, `last`) with the string `that`.
+   * \pre `first` and `last` point to characters within this string (including
+   *   the terminating null).
+   * \note Equivalent to
+   *   `replace(first - data(), last - first, that.data(), that.size())`
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      const Char* first,
+      const Char* last,
+      const BasicFixedString<Char, M>& that) noexcept(false) {
+    return replace(first - data_, last - first, that, 0u, that.size_);
+  }
+
+  /**
+   * Replace `this_count` characters starting from position `this_pos` with the
+   *   characters from string `that` starting at position `that_pos`.
+   * \pre `that_pos <= that.size()`
+   * \note Equivalent to
+   *   <tt>replace(this_pos, this_count, that.data() + that_pos,
+   *   that.size() - that_pos)</tt>
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos = 0u) noexcept(false) {
+    return replace(this_pos, this_count, that, that_pos, that.size_ - that_pos);
+  }
+
+  /**
+   * Replace `this_count` characters starting from position `this_pos` with
+   *   `that_count` characters from string `that` starting at position
+   *   `that_pos`.
+   * \pre `that_pos <= that.size() && that_count <= that.size() - that_pos`
+   * \note Equivalent to
+   *   `replace(this_pos, this_count, that.data() + that_pos, that_count)`
+   */
+  template <std::size_t M>
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos,
+      std::size_t that_count) noexcept(false) {
+    return *this = creplace(this_pos, this_count, that, that_pos, that_count);
+  }
+
+  /**
+   * Replace `this_count` characters starting from position `this_pos` with
+   *   the characters from the string literal `that`.
+   * \note Equivalent to
+   *   `replace(this_pos, this_count, that, strlen(that))`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const Char* that) noexcept(false) {
+    return replace(this_pos, this_count, that, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Replace the characters denoted by the half-open range [`first`,`last`) with
+   *   the characters from the string literal `that`.
+   * \pre `first` and `last` point to characters within this string (including
+   *   the terminating null).
+   * \note Equivalent to
+   *   `replace(first - data(), last - first, that, strlen(that))`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      const Char* first,
+      const Char* last,
+      const Char* that) noexcept(false) {
+    return replace(
+        first - data_, last - first, that, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Replace `this_count` characters starting from position `this_pos` with
+   *   `that_count` characters from the character sequence pointed to by `that`.
+   * \param this_pos The starting offset within `*this` of the first character
+   *   to be replaced.
+   * \param this_count The number of characters to be replaced. If `npos`,
+   *   it is treated as if `this_count` were `size() - this_pos`.
+   * \param that A pointer to the replacement string.
+   * \param that_count The number of characters in the replacement string.
+   * \pre `this_pos <= size() && this_count <= size() - this_pos`
+   * \pre `that` points to a contiguous sequence of at least `that_count`
+   *   characters
+   * \throw std::out_of_range on any of the following conditions:
+   *   - `this_pos > size()`
+   *   - `this_count > size() - this_pos`
+   *   - `size() - this_count + that_count > N`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const Char* that,
+      std::size_t that_count) noexcept(false) {
+    return *this = detail::fixedstring::Helper::replace_<Char>(
+               *this,
+               detail::fixedstring::checkOverflow(this_pos, size_),
+               detail::fixedstring::checkOverflowOrNpos(
+                   this_count, size_ - this_pos),
+               that,
+               0u,
+               that_count,
+               Indices{});
+  }
+
+  /**
+   * Replace `this_count` characters starting from position `this_pos` with
+   *   `that_count` characters `ch`.
+   * \note Equivalent to
+   *   `replace(this_pos, this_count, BasicFixedString{that_count, ch})`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      std::size_t that_count,
+      Char ch) noexcept(false) {
+    return replace(this_pos, this_count, BasicFixedString{that_count, ch});
+  }
+
+  /**
+   * Replace the characters denoted by the half-open range [`first`,`last`)
+   *   with `that_count` characters `ch`.
+   * \note Equivalent to
+   *   `replace(first - data(), last - first, BasicFixedString{that_count, ch})`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      const Char* first,
+      const Char* last,
+      std::size_t that_count,
+      Char ch) noexcept(false) {
+    return replace(
+        first - data_, last - first, BasicFixedString{that_count, ch});
+  }
+
+  /**
+   * Replace the characters denoted by the half-open range [`first`,`last`) with
+   *   the characters from the string literal `that`.
+   * \pre `first` and `last` point to characters within this string (including
+   *   the terminating null).
+   * \note Equivalent to
+   *   `replace(this_pos, this_count, il.begin(), il.size())`
+   */
+  FOLLY_CPP14_CONSTEXPR BasicFixedString& replace(
+      const Char* first,
+      const Char* last,
+      std::initializer_list<Char> il) noexcept(false) {
+    return replace(first - data_, last - first, il.begin(), il.size());
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Construct a new string by replacing `this_count` characters starting from
+   *   position `this_pos` within this string with the characters from string
+   *   `that` starting at position `that_pos`.
+   * \pre `that_pos <= that.size()`
+   * \note Equivalent to
+   *   <tt>creplace(this_pos, this_count, that, that_pos,
+   *   that.size() - that_pos)</tt>
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> creplace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos = 0u) const noexcept(false) {
+    return creplace(
+        this_pos,
+        this_count,
+        that,
+        that_pos,
+        that.size_ - detail::fixedstring::checkOverflow(that_pos, that.size_));
+  }
+
+  /**
+   * Construct a new string by replacing `this_count` characters starting from
+   *   position `this_pos` within this string with `that_count` characters from
+   *   string `that` starting at position `that_pos`.
+   * \param this_pos The starting offset within `*this` of the first character
+   *   to be replaced.
+   * \param this_count The number of characters to be replaced. If `npos`,
+   *   it is treated as if `this_count` were `size() - this_pos`.
+   * \param that A string that contains the replacement string.
+   * \param that_pos The offset to the first character in the replacement
+   *   string.
+   * \param that_count The number of characters in the replacement string.
+   * \pre `this_pos <= size() && this_count <= size() - this_pos`
+   * \pre `that_pos <= that.size() && that_count <= that.size() - that_pos`
+   * \post The size of the returned string is `size() - this_count + that_count`
+   * \note Equivalent to <tt>BasicFixedString<Char, N + M>{substr(0, this_pos) +
+   *    that.substr(that_pos, that_count) + substr(this_pos + this_count)}</tt>
+   * \throw std::out_of_range on any of the following conditions:
+   *   - `this_pos > size()`
+   *   - `this_count > size() - this_pos`
+   *   - `that_pos > that.size()`
+   *   - `that_count > that.size() - that_pos`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> creplace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos,
+      std::size_t that_count) const noexcept(false) {
+    return detail::fixedstring::Helper::replace_<Char>(
+        *this,
+        detail::fixedstring::checkOverflow(this_pos, size_),
+        detail::fixedstring::checkOverflowOrNpos(this_count, size_ - this_pos),
+        that,
+        detail::fixedstring::checkOverflow(that_pos, that.size_),
+        detail::fixedstring::checkOverflowOrNpos(
+            that_count, that.size_ - that_pos),
+        std::make_index_sequence<N + M>{});
+  }
+
+  /**
+   * Construct a new string by replacing the characters denoted by the half-open
+   *   range [`first`,`last`) within this string with the characters from string
+   *   `that` starting at position `that_pos`.
+   * \pre `that_pos <= that.size()`
+   * \note Equivalent to
+   *   <tt>creplace(first - data(), last - first, that, that_pos,
+   *   that.size() - that_pos)</tt>
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> creplace(
+      const Char* first,
+      const Char* last,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos = 0u) const noexcept(false) {
+    return creplace(
+        first - data_,
+        last - first,
+        that,
+        that_pos,
+        that.size_ - detail::fixedstring::checkOverflow(that_pos, that.size_));
+  }
+
+  /**
+   * Construct a new string by replacing the characters denoted by the half-open
+   *   range [`first`,`last`) within this string with the `that_count`
+   *   characters from string `that` starting at position `that_pos`.
+   * \note Equivalent to
+   *   <tt>creplace(first - data(), last - first, that, that_pos,
+   *   that_count)</tt>
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M> creplace(
+      const Char* first,
+      const Char* last,
+      const BasicFixedString<Char, M>& that,
+      std::size_t that_pos,
+      std::size_t that_count) const noexcept(false) {
+    return creplace(first - data_, last - first, that, that_pos, that_count);
+  }
+
+  /**
+   * Construct a new string by replacing `this_count` characters starting from
+   *   position `this_pos` within this string with `M-1` characters from
+   *   character array `that`.
+   * \pre `strlen(that) == M-1`
+   * \note Equivalent to
+   *   <tt>creplace(this_pos, this_count, that, 0, M - 1)</tt>
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u> creplace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const Char (&that)[M]) const noexcept(false) {
+    return creplace(this_pos, this_count, that, 0u, M - 1u);
+  }
+
+  /**
+   * Replace `this_count` characters starting from position `this_pos` with
+   *   `that_count` characters from the character array `that` starting at
+   *   position `that_pos`.
+   * \param this_pos The starting offset within `*this` of the first character
+   *   to be replaced.
+   * \param this_count The number of characters to be replaced. If `npos`,
+   *   it is treated as if `this_count` were `size() - this_pos`.
+   * \param that An array of characters containing the replacement string.
+   * \param that_pos The starting offset of the replacement string.
+   * \param that_count The number of characters in the replacement string.  If
+   *   `npos`, it is treated as if `that_count` were `M - 1 - that_pos`
+   * \pre `this_pos <= size() && this_count <= size() - this_pos`
+   * \pre `that_pos <= M - 1 && that_count <= M - 1 - that_pos`
+   * \post The size of the returned string is `size() - this_count + that_count`
+   * \note Equivalent to <tt>BasicFixedString<Char, N + M - 1>{
+   *    substr(0, this_pos) +
+   *    makeFixedString(that).substr(that_pos, that_count) +
+   *    substr(this_pos + this_count)}</tt>
+   * \throw std::out_of_range on any of the following conditions:
+   *   - `this_pos > size()`
+   *   - `this_count > size() - this_pos`
+   *   - `that_pos >= M`
+   *   - `that_count >= M - that_pos`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u> creplace(
+      std::size_t this_pos,
+      std::size_t this_count,
+      const Char (&that)[M],
+      std::size_t that_pos,
+      std::size_t that_count) const noexcept(false) {
+    return detail::fixedstring::Helper::replace_<Char>(
+        *this,
+        detail::fixedstring::checkOverflow(this_pos, size_),
+        detail::fixedstring::checkOverflowOrNpos(this_count, size_ - this_pos),
+        detail::fixedstring::checkNullTerminated(that),
+        detail::fixedstring::checkOverflow(that_pos, M - 1u),
+        detail::fixedstring::checkOverflowOrNpos(that_count, M - 1u - that_pos),
+        std::make_index_sequence<N + M - 1u>{});
+  }
+
+  /**
+   * Construct a new string by replacing the characters denoted by the half-open
+   *   range [`first`,`last`) within this string with the first `M-1`
+   *   characters from the character array `that`.
+   * \pre `strlen(that) == M-1`
+   * \note Equivalent to
+   *   <tt>creplace(first - data(), last - first, that, 0, M-1)</tt>
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u>
+  creplace(const Char* first, const Char* last, const Char (&that)[M]) const
+      noexcept(false) {
+    return creplace(first - data_, last - first, that, 0u, M - 1u);
+  }
+
+  /**
+   * Construct a new string by replacing the characters denoted by the half-open
+   *   range [`first`,`last`) within this string with the `that_count`
+   *   characters from the character array `that` starting at position
+   *   `that_pos`.
+   * \pre `strlen(that) == M-1`
+   * \note Equivalent to
+   *   `creplace(first - data(), last - first, that, that_pos, that_count)`
+   */
+  template <std::size_t M>
+  constexpr BasicFixedString<Char, N + M - 1u> creplace(
+      const Char* first,
+      const Char* last,
+      const Char (&that)[M],
+      std::size_t that_pos,
+      std::size_t that_count) const noexcept(false) {
+    return creplace(first - data_, last - first, that, that_pos, that_count);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+  * Copies `min(count, size())` characters starting from offset `0`
+  *   from this string into the buffer pointed to by `dest`.
+  * \return The number of characters copied.
+   */
+  FOLLY_CPP14_CONSTEXPR std::size_t copy(Char* dest, std::size_t count) const
+      noexcept {
+    return copy(dest, count, 0u);
+  }
+
+  /**
+   * Copies `min(count, size() - pos)` characters starting from offset `pos`
+   *   from this string into the buffer pointed to by `dest`.
+   * \pre `pos <= size()`
+   * \return The number of characters copied.
+   * \throw std::out_of_range if `pos > size()`
+   */
+  FOLLY_CPP14_CONSTEXPR std::size_t
+  copy(Char* dest, std::size_t count, std::size_t pos) const noexcept(false) {
+    detail::fixedstring::checkOverflow(pos, size_);
+    for (std::size_t i = 0u; i < count; ++i) {
+      if (i + pos == size_)
+        return size_;
+      dest[i] = data_[i + pos];
+    }
+    return count;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Resizes the current string.
+   * \note Equivalent to `resize(count, Char(0))`
+   */
+  FOLLY_CPP14_CONSTEXPR void resize(std::size_t count) noexcept(false) {
+    resize(count, Char(0));
+  }
+
+  /**
+   * Resizes the current string by setting the size to `count` and setting
+   *   `data()[count]` to `Char(0)`. If `count > old_size`, the characters
+   *   in the range [`old_size`,`count`) are set to `ch`.
+   */
+  FOLLY_CPP14_CONSTEXPR void resize(std::size_t count, Char ch) noexcept(
+      false) {
+    detail::fixedstring::checkOverflow(count, N);
+    if (count == size_) {
+    } else if (count < size_) {
+      size_ = count;
+      data_[size_] = Char(0);
+    } else {
+      for (; size_ < count; ++size_) {
+        data_[size_] = ch;
+      }
+      data_[size_] = Char(0);
+    }
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the first occurrence of the character sequence `that` in this string.
+   * \note Equivalent to `find(that.data(), 0, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find(const BasicFixedString<Char, M>& that) const
+      noexcept {
+    return find(that, 0u);
+  }
+
+  /**
+   * Finds the first occurrence of the character sequence `that` in this string,
+   *   starting at offset `pos`.
+   * \pre `pos <= size()`
+   * \note Equivalent to `find(that.data(), pos, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) {
+    return that.size_ <= size_ - detail::fixedstring::checkOverflow(pos, size_)
+        ? detail::fixedstring::find_(*this, that, pos, that.size_)
+        : npos;
+  }
+
+  /**
+   * Finds the first occurrence of the character sequence `that` in this string.
+   * \note Equivalent to `find(that.data(), 0, strlen(that))`
+   */
+  constexpr std::size_t find(const Char* that) const noexcept {
+    return find(that, 0u, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the first occurrence of the character sequence `that` in this string,
+   *   starting at offset `pos`.
+   * \pre `pos <= size()`
+   * \note Equivalent to `find(that.data(), pos, strlen(that))`
+   */
+  constexpr std::size_t find(const Char* that, std::size_t pos) const
+      noexcept(false) {
+    return find(that, pos, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the first occurrence of the first `count` characters in the buffer
+   *   pointed to by `that` in this string, starting at offset `pos`.
+   * \pre `pos <= size()`
+   * \pre `that` points to a buffer containing at least `count` contiguous
+   *   characters.
+   * \return The lowest offset `i` such that `i >= pos` and
+   *   `0 == strncmp(data() + i, that, count)`; or `npos` if there is no such
+   *   offset `i`.
+   * \throw std::out_of_range when `pos > size()`
+   */
+  constexpr std::size_t find(
+      const Char* that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return count <= size_ - detail::fixedstring::checkOverflow(pos, size_)
+        ? detail::fixedstring::find_(*this, that, pos, count)
+        : npos;
+  }
+
+  /**
+   * Finds the first occurrence of the character `ch` in this string.
+   * \note Equivalent to `find(&ch, 0, 1)`
+   */
+  constexpr std::size_t find(Char ch) const noexcept {
+    return find(ch, 0u);
+  }
+
+  /**
+   * Finds the first occurrence of the character character `c` in this string,
+   *   starting at offset `pos`.
+   * \pre `pos <= size()`
+   * \note Equivalent to `find(&ch, pos, 1)`
+   */
+  constexpr std::size_t find(Char ch, std::size_t pos) const noexcept(false) {
+    using A = const Char[1u];
+    return 0u == size_ - detail::fixedstring::checkOverflow(pos, size_)
+        ? npos
+        : detail::fixedstring::find_(*this, A{ch}, pos, 1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the last occurrence of characters in the string
+   *   `that` in this string.
+   * \note Equivalent to `rfind(that.data(), size(), that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t rfind(const BasicFixedString<Char, M>& that) const
+      noexcept {
+    return rfind(that, size_);
+  }
+
+  /**
+   * Finds the last occurrence of characters in the string
+   *   `that` in this string, starting at offset `pos`.
+   * \note Equivalent to `rfind(that.data(), pos, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t rfind(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) {
+    return that.size_ <= size_
+        ? detail::fixedstring::rfind_(
+              *this,
+              that,
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_),
+                  size_ - that.size_),
+              that.size_)
+        : npos;
+  }
+
+  /**
+   * Finds the last occurrence of characters in the buffer
+   *   pointed to by `that` in this string.
+   * \note Equivalent to `rfind(that, size(), strlen(that))`
+   */
+  constexpr std::size_t rfind(const Char* that) const noexcept {
+    return rfind(that, size_, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the last occurrence of characters in the buffer
+   *   pointed to by `that` in this string, starting at offset `pos`.
+   * \note Equivalent to `rfind(that, pos, strlen(that))`
+   */
+  constexpr std::size_t rfind(const Char* that, std::size_t pos) const
+      noexcept(false) {
+    return rfind(that, pos, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the last occurrence of the first `count` characters in the buffer
+   *   pointed to by `that` in this string, starting at offset `pos`.
+   * \pre `pos <= size()`
+   * \pre `that` points to a buffer containing at least `count` contiguous
+   *   characters.
+   * \return The largest offset `i` such that `i <= pos` and
+   *   `i + count <= size()` and `0 == strncmp(data() + i, that, count)`; or
+   *   `npos` if there is no such offset `i`.
+   * \throw std::out_of_range when `pos > size()`
+   */
+  constexpr std::size_t rfind(
+      const Char* that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return count <= size_
+        ? detail::fixedstring::rfind_(
+              *this,
+              that,
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_),
+                  size_ - count),
+              count)
+        : npos;
+  }
+
+  /**
+   * Finds the last occurrence of the character character `ch` in this string.
+   * \note Equivalent to `rfind(&ch, size(), 1)`
+   */
+  constexpr std::size_t rfind(Char ch) const noexcept {
+    return rfind(ch, size_);
+  }
+
+  /**
+   * Finds the last occurrence of the character character `ch` in this string,
+   *   starting at offset `pos`.
+   * \pre `pos <= size()`
+   * \note Equivalent to `rfind(&ch, pos, 1)`
+   */
+  constexpr std::size_t rfind(Char ch, std::size_t pos) const noexcept(false) {
+    using A = const Char[1u];
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::rfind_(
+              *this,
+              A{ch},
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the first occurrence of any character in `that` in this string.
+   * \note Equivalent to `find_first_of(that.data(), 0, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_first_of(
+      const BasicFixedString<Char, M>& that) const noexcept {
+    return find_first_of(that, 0u);
+  }
+
+  /**
+   * Finds the first occurrence of any character in `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_first_of(that.data(), pos, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_first_of(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) {
+    return size_ == detail::fixedstring::checkOverflow(pos, size_)
+        ? npos
+        : detail::fixedstring::find_first_of_(*this, that, pos, that.size_);
+  }
+
+  /**
+   * Finds the first occurrence of any character in the null-terminated
+   *   character sequence pointed to by `that` in this string.
+   * \note Equivalent to `find_first_of(that, 0, strlen(that))`
+   */
+  constexpr std::size_t find_first_of(const Char* that) const noexcept {
+    return find_first_of(that, 0u, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the first occurrence of any character in the null-terminated
+   *   character sequence pointed to by `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_first_of(that, pos, strlen(that))`
+   */
+  constexpr std::size_t find_first_of(const Char* that, std::size_t pos) const
+      noexcept(false) {
+    return find_first_of(that, pos, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the first occurrence of any character in the first `count` characters
+   *   in the buffer pointed to by `that` in this string, starting at offset
+   *  `pos`.
+   * \pre `pos <= size()`
+   * \pre `that` points to a buffer containing at least `count` contiguous
+   *   characters.
+   * \return The smallest offset `i` such that `i >= pos` and
+   *   `std::find(that, that+count, at(i)) != that+count`; or
+   *   `npos` if there is no such offset `i`.
+   * \throw std::out_of_range when `pos > size()`
+   */
+  constexpr std::size_t find_first_of(
+      const Char* that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return size_ == detail::fixedstring::checkOverflow(pos, size_)
+        ? npos
+        : detail::fixedstring::find_first_of_(*this, that, pos, count);
+  }
+
+  /**
+   * Finds the first occurrence of `ch` in this string.
+   * \note Equivalent to `find_first_of(&ch, 0, 1)`
+   */
+  constexpr std::size_t find_first_of(Char ch) const noexcept {
+    return find_first_of(ch, 0u);
+  }
+
+  /**
+   * Finds the first occurrence of `ch` in this string,
+   *   starting at offset `pos`.
+   * \note Equivalent to `find_first_of(&ch, pos, 1)`
+   */
+  constexpr std::size_t find_first_of(Char ch, std::size_t pos) const
+      noexcept(false) {
+    using A = const Char[1u];
+    return size_ == detail::fixedstring::checkOverflow(pos, size_)
+        ? npos
+        : detail::fixedstring::find_first_of_(*this, A{ch}, pos, 1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the first occurrence of any character not in `that` in this string.
+   * \note Equivalent to `find_first_not_of(that.data(), 0, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_first_not_of(
+      const BasicFixedString<Char, M>& that) const noexcept {
+    return find_first_not_of(that, 0u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the first occurrence of any character not in `that` in this string.
+   * \note Equivalent to `find_first_not_of(that.data(), 0, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_first_not_of(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) {
+    return size_ == detail::fixedstring::checkOverflow(pos, size_)
+        ? npos
+        : detail::fixedstring::find_first_not_of_(*this, that, pos, that.size_);
+  }
+
+  /**
+   * Finds the first occurrence of any character not in the null-terminated
+   *   character sequence pointed to by `that` in this string.
+   * \note Equivalent to `find_first_not_of(that, 0, strlen(that))`
+   */
+  constexpr std::size_t find_first_not_of(const Char* that) const noexcept {
+    return find_first_not_of(that, 0u, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the first occurrence of any character not in the null-terminated
+   *   character sequence pointed to by `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_first_not_of(that, pos, strlen(that))`
+   */
+  constexpr std::size_t find_first_not_of(const Char* that, std::size_t pos)
+      const noexcept(false) {
+    return find_first_not_of(that, pos, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the first occurrence of any character not in the first `count`
+   *   characters in the buffer pointed to by `that` in this string, starting at
+   *   offset `pos`.
+   * \pre `pos <= size()`
+   * \pre `that` points to a buffer containing at least `count` contiguous
+   *   characters.
+   * \return The smallest offset `i` such that `i >= pos` and
+   *   `std::find(that, that+count, at(i)) == that+count`; or
+   *   `npos` if there is no such offset `i`.
+   * \throw std::out_of_range when `pos > size()`
+   */
+  constexpr std::size_t find_first_not_of(
+      const Char* that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return size_ == detail::fixedstring::checkOverflow(pos, size_)
+        ? npos
+        : detail::fixedstring::find_first_not_of_(*this, that, pos, count);
+  }
+
+  /**
+   * Finds the first occurrence of any character other than `ch` in this string.
+   * \note Equivalent to `find_first_not_of(&ch, 0, 1)`
+   */
+  constexpr std::size_t find_first_not_of(Char ch) const noexcept {
+    return find_first_not_of(ch, 0u);
+  }
+
+  /**
+   * Finds the first occurrence of any character other than `ch` in this string,
+   *   starting at offset `pos`.
+   * \note Equivalent to `find_first_not_of(&ch, pos, 1)`
+   */
+  constexpr std::size_t find_first_not_of(Char ch, std::size_t pos) const
+      noexcept(false) {
+    using A = const Char[1u];
+    return 1u <= size_ - detail::fixedstring::checkOverflow(pos, size_)
+        ? detail::fixedstring::find_first_not_of_(*this, A{ch}, pos, 1u)
+        : npos;
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the last occurrence of any character in `that` in this string.
+   * \note Equivalent to `find_last_of(that.data(), size(), that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_last_of(
+      const BasicFixedString<Char, M>& that) const noexcept {
+    return find_last_of(that, size_);
+  }
+
+  /**
+   * Finds the last occurrence of any character in `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_last_of(that.data(), pos, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_last_of(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) {
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::find_last_of_(
+              *this,
+              that,
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              that.size_);
+  }
+
+  /**
+   * Finds the last occurrence of any character in the null-terminated
+   *   character sequence pointed to by `that` in this string.
+   * \note Equivalent to `find_last_of(that, size(), strlen(that))`
+   */
+  constexpr std::size_t find_last_of(const Char* that) const noexcept {
+    return find_last_of(that, size_, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the last occurrence of any character in the null-terminated
+   *   character sequence pointed to by `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_last_of(that, pos, strlen(that))`
+   */
+  constexpr std::size_t find_last_of(const Char* that, std::size_t pos) const
+      noexcept(false) {
+    return find_last_of(that, pos, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the last occurrence of any character in the first `count` characters
+   *   in the buffer pointed to by `that` in this string, starting at offset
+   *  `pos`.
+   * \pre `pos <= size()`
+   * \pre `that` points to a buffer containing at least `count` contiguous
+   *   characters.
+   * \return The largest offset `i` such that `i <= pos` and
+   *   `i < size()` and `std::find(that, that+count, at(i)) != that+count`; or
+   *   `npos` if there is no such offset `i`.
+   * \throw std::out_of_range when `pos > size()`
+   */
+  constexpr std::size_t find_last_of(
+      const Char* that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::find_last_of_(
+              *this,
+              that,
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              count);
+  }
+
+  /**
+   * Finds the last occurrence of `ch` in this string.
+   * \note Equivalent to `find_last_of(&ch, size(), 1)`
+   */
+  constexpr std::size_t find_last_of(Char ch) const noexcept {
+    return find_last_of(ch, size_);
+  }
+
+  /**
+   * Finds the last occurrence of `ch` in this string,
+   *   starting at offset `pos`.
+   * \note Equivalent to `find_last_of(&ch, pos, 1)`
+   */
+  constexpr std::size_t find_last_of(Char ch, std::size_t pos) const
+      noexcept(false) {
+    using A = const Char[1u];
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::find_last_of_(
+              *this,
+              A{ch},
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Finds the last occurrence of any character not in `that` in this string.
+   * \note Equivalent to `find_last_not_of(that.data(), size(), that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_last_not_of(
+      const BasicFixedString<Char, M>& that) const noexcept {
+    return find_last_not_of(that, size_);
+  }
+
+  /**
+   * Finds the last occurrence of any character not in `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_last_not_of(that.data(), pos, that.size())`
+   */
+  template <std::size_t M>
+  constexpr std::size_t find_last_not_of(
+      const BasicFixedString<Char, M>& that,
+      std::size_t pos) const noexcept(false) {
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::find_last_not_of_(
+              *this,
+              that,
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              that.size_);
+  }
+
+  /**
+   * Finds the last occurrence of any character not in the null-terminated
+   *   character sequence pointed to by `that` in this string.
+   * \note Equivalent to `find_last_not_of(that, size(), strlen(that))`
+   */
+  constexpr std::size_t find_last_not_of(const Char* that) const noexcept {
+    return find_last_not_of(that, size_, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the last occurrence of any character not in the null-terminated
+   *   character sequence pointed to by `that` in this string,
+   *   starting at offset `pos`
+   * \note Equivalent to `find_last_not_of(that, pos, strlen(that))`
+   */
+  constexpr std::size_t find_last_not_of(const Char* that, std::size_t pos)
+      const noexcept(false) {
+    return find_last_not_of(that, pos, folly::constexpr_strlen(that));
+  }
+
+  /**
+   * Finds the last occurrence of any character not in the first `count`
+   *   characters in the buffer pointed to by `that` in this string, starting at
+   *   offset `pos`.
+   * \pre `pos <= size()`
+   * \pre `that` points to a buffer containing at least `count` contiguous
+   *   characters.
+   * \return The largest offset `i` such that `i <= pos` and
+   *   `i < size()` and `std::find(that, that+count, at(i)) == that+count`; or
+   *   `npos` if there is no such offset `i`.
+   * \throw std::out_of_range when `pos > size()`
+   */
+  constexpr std::size_t find_last_not_of(
+      const Char* that,
+      std::size_t pos,
+      std::size_t count) const noexcept(false) {
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::find_last_not_of_(
+              *this,
+              that,
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              count);
+  }
+
+  /**
+   * Finds the last occurrence of any character other than `ch` in this string.
+   * \note Equivalent to `find_last_not_of(&ch, size(), 1)`
+   */
+  constexpr std::size_t find_last_not_of(Char ch) const noexcept {
+    return find_last_not_of(ch, size_);
+  }
+
+  /**
+   * Finds the last occurrence of any character other than `ch` in this string,
+   *   starting at offset `pos`.
+   * \note Equivalent to `find_last_not_of(&ch, pos, 1)`
+   */
+  constexpr std::size_t find_last_not_of(Char ch, std::size_t pos) const
+      noexcept(false) {
+    using A = const Char[1u];
+    return 0u == size_
+        ? npos
+        : detail::fixedstring::find_last_not_of_(
+              *this,
+              A{ch},
+              folly::constexpr_min(
+                  detail::fixedstring::checkOverflow(pos, size_), size_ - 1u),
+              1u);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Asymmetric relational operators
+   */
+  friend constexpr bool operator==(
+      const Char* a,
+      const BasicFixedString& b) noexcept {
+    return detail::fixedstring::equal_(
+        a, folly::constexpr_strlen(a), b, b.size());
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr bool operator==(
+      const BasicFixedString& a,
+      const Char* b) noexcept {
+    return b == a;
+  }
+
+  friend constexpr bool operator!=(
+      const Char* a,
+      const BasicFixedString& b) noexcept {
+    return !(a == b);
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr bool operator!=(
+      const BasicFixedString& a,
+      const Char* b) noexcept {
+    return !(b == a);
+  }
+
+  friend constexpr bool operator<(
+      const Char* a,
+      const BasicFixedString& b) noexcept {
+    return detail::fixedstring::Cmp::LT ==
+        detail::fixedstring::compare_(
+               a, 0u, folly::constexpr_strlen(a), b, 0u, b.size_);
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr bool operator<(
+      const BasicFixedString& a,
+      const Char* b) noexcept {
+    return detail::fixedstring::Cmp::LT ==
+        detail::fixedstring::compare_(
+               a, 0u, a.size_, b, 0u, folly::constexpr_strlen(b));
+  }
+
+  friend constexpr bool operator>(
+      const Char* a,
+      const BasicFixedString& b) noexcept {
+    return b < a;
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr bool operator>(
+      const BasicFixedString& a,
+      const Char* b) noexcept {
+    return b < a;
+  }
+
+  friend constexpr bool operator<=(
+      const Char* a,
+      const BasicFixedString& b) noexcept {
+    return !(b < a);
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr bool operator<=(
+      const BasicFixedString& a,
+      const Char* b) noexcept {
+    return !(b < a);
+  }
+
+  friend constexpr bool operator>=(
+      const Char* a,
+      const BasicFixedString& b) noexcept {
+    return !(a < b);
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr bool operator>=(
+      const BasicFixedString& a,
+      const Char* b) noexcept {
+    return !(a < b);
+  }
+
+  /** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+   * Asymmetric concatenation
+   */
+  template <std::size_t M>
+  friend constexpr BasicFixedString<Char, N + M - 1u> operator+(
+      const Char (&a)[M],
+      const BasicFixedString& b) noexcept {
+    return detail::fixedstring::Helper::concat_<Char>(
+        detail::fixedstring::checkNullTerminated(a),
+        b,
+        std::make_index_sequence<N + M - 1u>{});
+  }
+
+  /**
+   * \overload
+   */
+  template <std::size_t M>
+  friend constexpr BasicFixedString<Char, N + M - 1u> operator+(
+      const BasicFixedString& a,
+      const Char (&b)[M]) noexcept {
+    return detail::fixedstring::Helper::concat_<Char>(
+        a,
+        detail::fixedstring::checkNullTerminated(b),
+        std::make_index_sequence<N + M - 1u>{});
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr BasicFixedString<Char, N + 1u> operator+(
+      Char a,
+      const BasicFixedString& b) noexcept {
+    using A = const Char[2u];
+    return detail::fixedstring::Helper::concat_<Char>(
+        A{a, Char(0)}, b, std::make_index_sequence<N + 1u>{});
+  }
+
+  /**
+   * \overload
+   */
+  friend constexpr BasicFixedString<Char, N + 1u> operator+(
+      const BasicFixedString& a,
+      Char b) noexcept {
+    using A = const Char[2u];
+    return detail::fixedstring::Helper::concat_<Char>(
+        a, A{b, Char(0)}, std::make_index_sequence<N + 1u>{});
+  }
+};
+
+/** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+ * Symmetric relational operators
+ */
+template <class Char, std::size_t A, std::size_t B>
+constexpr bool operator==(
+    const BasicFixedString<Char, A>& a,
+    const BasicFixedString<Char, B>& b) noexcept {
+  return detail::fixedstring::equal_(a, a.size(), b, b.size());
+}
+
+template <class Char, std::size_t A, std::size_t B>
+constexpr bool operator!=(
+    const BasicFixedString<Char, A>& a,
+    const BasicFixedString<Char, B>& b) {
+  return !(a == b);
+}
+
+template <class Char, std::size_t A, std::size_t B>
+constexpr bool operator<(
+    const BasicFixedString<Char, A>& a,
+    const BasicFixedString<Char, B>& b) noexcept {
+  return detail::fixedstring::Cmp::LT ==
+      detail::fixedstring::compare_(a, 0u, a.size(), b, 0u, b.size());
+}
+
+template <class Char, std::size_t A, std::size_t B>
+constexpr bool operator>(
+    const BasicFixedString<Char, A>& a,
+    const BasicFixedString<Char, B>& b) noexcept {
+  return b < a;
+}
+
+template <class Char, std::size_t A, std::size_t B>
+constexpr bool operator<=(
+    const BasicFixedString<Char, A>& a,
+    const BasicFixedString<Char, B>& b) noexcept {
+  return !(b < a);
+}
+
+template <class Char, std::size_t A, std::size_t B>
+constexpr bool operator>=(
+    const BasicFixedString<Char, A>& a,
+    const BasicFixedString<Char, B>& b) noexcept {
+  return !(a < b);
+}
+
+/** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+ * Symmetric concatenation
+ */
+template <class Char, std::size_t N, std::size_t M>
+constexpr BasicFixedString<Char, N + M> operator+(
+    const BasicFixedString<Char, N>& a,
+    const BasicFixedString<Char, M>& b) noexcept {
+  return detail::fixedstring::Helper::concat_<Char>(
+      a, b, std::make_index_sequence<N + M>{});
+}
+
+/** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+ * Construct a `BasicFixedString` object from a null-terminated array of
+ * characters. The capacity and size of the string will be equal to one less
+ * than the size of the array.
+ * \pre `a` contains no embedded null characters.
+ * \pre `a[N-1] == Char(0)`
+ * \post For a returned string `s`, `s[i]==a[i]` for every `i` in [`0`,`N-1`].
+ */
+template <class Char, std::size_t N>
+constexpr BasicFixedString<Char, N - 1u> makeFixedString(
+    const Char (&a)[N]) noexcept {
+  return {a};
+}
+
+/** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **
+ * Swap function
+ */
+template <class Char, std::size_t N>
+FOLLY_CPP14_CONSTEXPR void swap(
+    BasicFixedString<Char, N>& a,
+    BasicFixedString<Char, N>& b) noexcept {
+  a.swap(b);
+}
+
+inline namespace Literals {
+inline namespace StringLiterals {
+inline namespace {
+// "const std::size_t&" is so that folly::npos has the same address in every
+// translation unit. This is to avoid potential violations of the ODR.
+constexpr const std::size_t& npos = detail::fixedstring::FixedStringBase::npos;
+}
+
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"
+#pragma GCC diagnostic ignored "-Wgnu-string-literal-operator-template"
+
+/** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** *
+ * User-defined literals for creating FixedString objects from string literals
+ * on the compilers that support it.
+ *
+ * \par Example:
+ * \par
+ * \code
+ * using namespace folly::StringLiterals;
+ * constexpr auto hello = "hello world!"_fs;
+ * \endcode
+ *
+ * \note This requires a GNU compiler extension
+ *   (-Wgnu-string-literal-operator-template) supported by clang and gcc,
+ *   proposed for standardization in
+ *   <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0424r0.pdf>.
+ *   \par
+ *   For portable code, prefer the suffixes `_fs4`, `_fs8`, `_fs16`, `_fs32`,
+ *   `_fs64`, and `_fs128` for creating instances of types `FixedString<4>`,
+ *   `FixedString<8>`, `FixedString<16>`, etc.
+ */
+template <class Char, Char... Cs>
+constexpr BasicFixedString<Char, sizeof...(Cs)> operator"" _fs() noexcept {
+  using A = const Char[sizeof...(Cs) + 1u];
+  // The `+` in `+A{etc}` forces the array type to decay to a pointer
+  return {+A{Cs..., Char(0)}, sizeof...(Cs)};
+}
+
+#pragma GCC diagnostic pop
+#endif
+
+#define FOLLY_DEFINE_FIXED_STRING_UDL(N)                     \
+  constexpr FixedString<N> operator"" _fs##N(                \
+      const char* that, std::size_t count) noexcept(false) { \
+    return {that, count};                                    \
+  }                                                          \
+/**/
+
+// Define UDLs _fs4, _fs8, _fs16, etc for FixedString<[4, 8, 16, ...]>
+FOLLY_DEFINE_FIXED_STRING_UDL(4)
+FOLLY_DEFINE_FIXED_STRING_UDL(8)
+FOLLY_DEFINE_FIXED_STRING_UDL(16)
+FOLLY_DEFINE_FIXED_STRING_UDL(32)
+FOLLY_DEFINE_FIXED_STRING_UDL(64)
+FOLLY_DEFINE_FIXED_STRING_UDL(128)
+
+#undef FOLLY_DEFINE_FIXED_STRING_UDL
+}
+}
+
+// TODO:
+// // numeric conversions:
+// template <std::size_t N>
+// constexpr int stoi(const FixedString<N>& str, int base = 10);
+// template <std::size_t N>
+// constexpr unsigned stou(const FixedString<N>& str, int base = 10);
+// template <std::size_t N>
+// constexpr long stol(const FixedString<N>& str, int base = 10);
+// template <std::size_t N>
+// constexpr unsigned long stoul(const FixedString<N>& str, int base = 10;
+// template <std::size_t N>
+// constexpr long long stoll(const FixedString<N>& str, int base = 10);
+// template <std::size_t N>
+// constexpr unsigned long long stoull(const FixedString<N>& str,
+// int base = 10);
+// template <std::size_t N>
+// constexpr float stof(const FixedString<N>& str);
+// template <std::size_t N>
+// constexpr double stod(const FixedString<N>& str);
+// template <std::size_t N>
+// constexpr long double stold(const FixedString<N>& str);
+// template <int val>
+// constexpr FixedString</*...*/> to_fixed_string_i() noexcept;
+// template <unsigned val>
+// constexpr FixedString</*...*/> to_fixed_string_u() noexcept;
+// template <long val>
+// constexpr FixedString</*...*/> to_fixed_string_l() noexcept;
+// template <unsigned long val>
+// constexpr FixedString</*...*/> to_fixed_string_ul() noexcept;
+// template <long long val>
+// constexpr FixedString</*...*/> to_fixed_string_ll() noexcept
+// template <unsigned long long val>
+// constexpr FixedString</*...*/> to_fixed_string_ull() noexcept;
+}
diff --git a/folly/Makefile.am b/folly/Makefile.am
index 6aea6dd1..23346fd0 100644
--- a/folly/Makefile.am
+++ b/folly/Makefile.am
@@ -144,6 +144,7 @@ nobase_follyinclude_HEADERS = \
 	File.h \
 	FileUtil.h \
 	Fingerprint.h \
+	FixedString.h \
 	folly-config.h \
 	Foreach.h \
 	FormatArg.h \
diff --git a/folly/test/FixedStringTest.cpp b/folly/test/FixedStringTest.cpp
new file mode 100644
index 00000000..3dfb414b
--- /dev/null
+++ b/folly/test/FixedStringTest.cpp
@@ -0,0 +1,653 @@
+/*
+ * Copyright 2016 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+//
+// Author: eniebler@fb.com
+
+#include <folly/FixedString.h>
+#include <folly/portability/GTest.h>
+
+#define FS(x) ::folly::makeFixedString(x)
+using namespace folly::StringLiterals;
+
+TEST(FixedStringExamples, Examples) {
+  // Example from the docs:
+  using namespace folly;
+  constexpr auto hello = makeFixedString("hello"); // a FixedString<5>
+  constexpr auto world = makeFixedString("world"); // another FixedString<5>
+  constexpr auto hello_world = hello + ' ' + world + '!';
+  static_assert(hello_world == "hello world!", "w00t");
+  EXPECT_STREQ("hello world!", hello_world.c_str());
+
+  FixedString<10> test{"****"};
+  test.replace(1, 2, "!!!!");
+  EXPECT_STREQ("*!!!!*", test.c_str());
+  static_assert(makeFixedString("****").creplace(1, 2, "!!!!") == "*!!!!*", "");
+}
+
+TEST(FixedStringCtorTest, Default) {
+  constexpr folly::FixedString<42> s{};
+  static_assert(s[0] == '\0', "");
+  static_assert(s.size() == 0u, "");
+
+  constexpr auto s2 = s;
+  static_assert(s2[0] == '\0', "");
+  static_assert(s2.size() == 0u, "");
+}
+
+TEST(FixedStringCtorTest, FromLiterals) {
+  constexpr folly::FixedString<42> s{"hello world"};
+  constexpr folly::FixedString<11> s2{"hello world"};
+  static_assert(s2[0] == 'h', "");
+  static_assert(s2[10] == 'd', "");
+  static_assert(s2[11] == '\0', "");
+
+  // Does not compile, hurray! :-)
+  // constexpr char a[1] = {'a'};
+  // constexpr folly::FixedString<10> s3(a);
+}
+
+TEST(FixedStringCtorTest, FromPtrAndLength) {
+  constexpr folly::FixedString<11> s{"hello world", 11};
+  static_assert(s[0] == 'h', "");
+  static_assert(s[10] == 'd', "");
+  static_assert(s[11] == '\0', "");
+  static_assert(s.size() == 11u, "");
+
+  constexpr folly::FixedString<5> s2{"hello world", 5};
+  static_assert(s2[0] == 'h', "");
+  static_assert(s2[4] == 'o', "");
+  static_assert(s2[5] == '\0', "");
+  static_assert(s2.size() == 5u, "");
+
+  constexpr folly::FixedString<20> s3{"hello world", 5};
+  static_assert(s2[0] == 'h', "");
+  static_assert(s2[4] == 'o', "");
+  static_assert(s2[5] == '\0', "");
+  static_assert(s2.size() == 5u, "");
+
+  static_assert("hello" == s3, "");
+  static_assert(s3 == "hello", "");
+  static_assert(s3 == s2, "");
+  static_assert("hell" != s3, "");
+  static_assert(s3 != "helloooo", "");
+  static_assert(!(s3 != s2), "");
+}
+
+TEST(FixedStringCtorTest, FromStringAndOffset) {
+  constexpr folly::FixedString<11> s{"hello world"};
+  constexpr folly::FixedString<5> s2{s, 6u, npos};
+  static_assert(s2 == "world", "");
+  constexpr folly::FixedString<0> s3{s, 11u, npos};
+  static_assert(s3 == "", "");
+  // Out of bounds offset, does not compile
+  // constexpr folly::FixedString<0> s4{s, 12};
+}
+
+TEST(FixedStringCtorTest, FromStringOffsetAndCount) {
+  constexpr folly::FixedString<11> s{"hello world"};
+  constexpr folly::FixedString<4> s2{s, 6u, 4u};
+  static_assert(s2 == "worl", "");
+  constexpr folly::FixedString<5> s3{s, 6u, 5u};
+  static_assert(s3 == "world", "");
+  // Out of bounds count, does not compile:
+  // constexpr folly::FixedString<5> s4{s, 6, 6};
+}
+
+TEST(FixedStringCtorTest, FromInitializerList) {
+  constexpr folly::FixedString<11> s{
+      'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'};
+  static_assert(s == "hello world", "");
+  // Out of bounds count, does not compile:
+  // constexpr folly::FixedString<10> s{
+  //     {'h','e','l','l','o',' ','w','o','r','l','d'}};
+}
+
+TEST(FixedStringCtorTest, FromUDL) {
+  using namespace folly::Literals;
+#if defined(__GNUC__)
+  constexpr auto x = "hello"_fs;
+  static_assert(
+      std::is_same<decltype(x), const folly::FixedString<5>>::value, "");
+  static_assert(x[0] == 'h', "");
+  static_assert(x[1] == 'e', "");
+  static_assert(x[2] == 'l', "");
+  static_assert(x[3] == 'l', "");
+  static_assert(x[4] == 'o', "");
+  static_assert(x[5] == '\0', "");
+  static_assert(x.size() == 5u, "");
+#endif
+
+  constexpr auto y = "goodbye"_fs8;
+  static_assert(
+      std::is_same<decltype(y), const folly::FixedString<8>>::value, "");
+  static_assert(y.size() == 7u, "");
+  static_assert(y == "goodbye", "");
+
+  constexpr auto z = "now is the time for all good llamas"_fs64;
+  static_assert(
+      std::is_same<decltype(z), const folly::FixedString<64>>::value, "");
+  static_assert(z.size() == 35u, "");
+  static_assert(z == "now is the time for all good llamas", "");
+}
+
+TEST(FixedStringConcatTest, FromStringAndLiteral) {
+  constexpr folly::FixedString<42> s{"hello world"};
+  constexpr auto res = s + "!!!";
+  static_assert(res.size() == 14u, "");
+  static_assert(res == "hello world!!!", "");
+}
+
+TEST(FixedStringConcatTest, FromTwoStrings) {
+  constexpr folly::FixedString<42> s{"hello world"};
+  constexpr auto res = s + "!!!";
+  static_assert(res.size() == 14u, "");
+  static_assert(res == "hello world!!!", "");
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr folly::FixedString<20> constexpr_swap_test() {
+  folly::FixedString<10> tmp1{"hello"}, tmp2{"world!"};
+  tmp2.swap(tmp1);
+  return tmp1 + tmp2;
+}
+
+TEST(FixedStringSwapTest, ConstexprSwap) {
+  static_assert(constexpr_swap_test() == "world!hello", "");
+}
+#endif
+
+TEST(FixedStringSwapTest, RuntimeSwap) {
+  folly::FixedString<10> tmp1{"hello"}, tmp2{"world!"};
+  tmp2.swap(tmp1);
+  EXPECT_STREQ((tmp1 + tmp2).c_str(), "world!hello");
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr folly::FixedString<10> constexpr_assign_string_test_1() {
+  folly::FixedString<10> tmp1, tmp2{"world!"};
+  tmp1 = tmp2;
+  return tmp1;
+}
+constexpr folly::FixedString<10> constexpr_assign_string_test_2() {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp.assign("hello"_fs8);
+  return tmp;
+}
+constexpr folly::FixedString<10> constexpr_assign_string_test_3() {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp.assign("goodbye"_fs8, 3u, 2u);
+  return tmp;
+}
+constexpr folly::FixedString<10> constexpr_assign_string_test_4() {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp.assign("goodbye"_fs8, 3u, npos);
+  return tmp;
+}
+
+TEST(FixedStringAssignTest, ConstexprAssignString) {
+  static_assert(constexpr_assign_string_test_1() == "world!", "");
+  static_assert(constexpr_assign_string_test_2() == "hello", "");
+  static_assert(constexpr_assign_string_test_3() == "db", "");
+  static_assert(constexpr_assign_string_test_4() == "dbye", "");
+}
+#endif
+
+TEST(FixedStringAssignTest, RuntimeAssignString) {
+  folly::FixedString<10> tmp1, tmp2{"world!"};
+  tmp1 = tmp2;
+  EXPECT_STREQ(tmp1.c_str(), "world!");
+  tmp1.assign("goodbye"_fs8);
+  EXPECT_STREQ("goodbye", tmp1.c_str());
+  tmp1.assign("goodbye"_fs8, 3u, npos);
+  EXPECT_STREQ("dbye", tmp1.c_str());
+  tmp1.assign("goodbye"_fs8, 3u, 3u);
+  EXPECT_STREQ("dby", tmp1.c_str());
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr folly::FixedString<10> constexpr_assign_literal_test_1() {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp = "hello";
+  // Not null-terminated, does not compile:
+  // using C = const char[1];
+  // tmp = C{'a'};
+  return tmp;
+}
+constexpr folly::FixedString<10> constexpr_assign_literal_test_2() {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp.assign("hello");
+  return tmp;
+}
+constexpr folly::FixedString<10> constexpr_assign_literal_test_3() {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp.assign("goodbye", 4u);
+  return tmp;
+}
+
+TEST(FixedStringAssignTest, ConstexprAssignLiteral) {
+  static_assert(constexpr_assign_literal_test_1() == "hello", "");
+  static_assert(constexpr_assign_literal_test_2() == "hello", "");
+  static_assert(constexpr_assign_literal_test_3() == "good", "");
+}
+#endif
+
+TEST(FixedStringAssignTest, RuntimeAssignLiteral) {
+  folly::FixedString<10> tmp{"aaaaaaaaaa"};
+  tmp = "hello";
+  EXPECT_STREQ("hello", tmp.c_str());
+  tmp.assign("goodbye");
+  EXPECT_STREQ("goodbye", tmp.c_str());
+  tmp.assign("goodbye", 4u);
+  EXPECT_STREQ("good", tmp.c_str());
+}
+
+TEST(FixedStringIndexTest, Index) {
+  constexpr folly::FixedString<11> digits{"0123456789"};
+  static_assert(digits[0] == '0', "");
+  static_assert(digits[1] == '1', "");
+  static_assert(digits[2] == '2', "");
+  static_assert(digits[9] == '9', "");
+  static_assert(digits[10] == '\0', "");
+  #ifdef NDEBUG
+  // This should be allowed and work in constexpr mode since the internal array
+  // is actually big enough and op[] does no parameter validation:
+  static_assert(digits[11] == '\0', "");
+  #endif
+
+  static_assert(digits.at(0) == '0', "");
+  static_assert(digits.at(1) == '1', "");
+  static_assert(digits.at(2) == '2', "");
+  static_assert(digits.at(9) == '9', "");
+  static_assert(digits.at(10) == '\0', "");
+  EXPECT_THROW(digits.at(11), std::out_of_range);
+}
+
+TEST(FixedStringCompareTest, Compare) {
+  constexpr folly::FixedString<10> tmp1{"aaaaaaaaaa"};
+  constexpr folly::FixedString<12> tmp2{"aaaaaaaaaba"};
+  static_assert(-1 == tmp1.compare(tmp2), "");
+  static_assert(1 == tmp2.compare(tmp1), "");
+  static_assert(0 == tmp2.compare(tmp2), "");
+  static_assert(tmp1 < tmp2, "");
+  static_assert(tmp1 <= tmp2, "");
+  static_assert(tmp2 > tmp1, "");
+  static_assert(tmp2 >= tmp1, "");
+  static_assert(tmp2 == tmp2, ""); // @nolint
+  static_assert(tmp2 <= tmp2, ""); // @nolint
+  static_assert(tmp2 >= tmp2, ""); // @nolint
+  static_assert(!(tmp2 < tmp2), "");
+  static_assert(!(tmp2 > tmp2), "");
+
+  constexpr folly::FixedString<10> tmp3{"aaa"};
+  constexpr folly::FixedString<12> tmp4{"aaaa"};
+  static_assert(-1 == tmp3.compare(tmp4), "");
+  static_assert(1 == tmp4.compare(tmp3), "");
+  static_assert(tmp3 < tmp4, "");
+  static_assert(tmp3 <= tmp4, "");
+  static_assert(tmp4 > tmp3, "");
+  static_assert(tmp4 >= tmp3, "");
+  static_assert(tmp3 < "aaaa", "");
+  static_assert(tmp3 <= "aaaa", "");
+  static_assert(!(tmp3 == tmp4), "");
+  static_assert(tmp3 != tmp4, "");
+  static_assert("aaaa" > tmp3, "");
+  static_assert("aaaa" >= tmp3, "");
+  static_assert("aaaa" != tmp3, "");
+  static_assert("aaa" == tmp3, "");
+  static_assert(tmp3 != "aaaa", "");
+  static_assert(tmp3 == "aaa", "");
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr folly::FixedString<20> constexpr_append_string_test() {
+  folly::FixedString<20> a{"hello"}, b{"X world!"};
+  a.append(1u, ' ');
+  a.append(b, 2u, 5u);
+  a.append(b, 7u, 1u);
+  return a;
+}
+
+TEST(FixedStringAssignTest, ConstexprAppendString) {
+  static_assert(constexpr_append_string_test() == "hello world!", "");
+}
+#endif
+
+TEST(FixedStringAssignTest, RuntimeAppendString) {
+  folly::FixedString<20> a{"hello"}, b{"X world!"};
+  a.append(1u, ' ');
+  a.append(b, 2u, 5u);
+  a.append(b, 7u, 1u);
+  EXPECT_STREQ("hello world!", a.c_str());
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr folly::FixedString<20> constexpr_append_literal_test() {
+  folly::FixedString<20> a{"hello"};
+  a.append(1u, ' ');
+  a.append("X world!" + 2u, 5u);
+  a.append("X world!" + 7u);
+  return a;
+}
+
+TEST(FixedStringAssignTest, ConstexprAppendLiteral) {
+  static_assert(constexpr_append_literal_test() == "hello world!", "");
+}
+#endif
+
+TEST(FixedStringAssignTest, RuntimeAppendLiteral) {
+  folly::FixedString<20> a{"hello"};
+  a.append(1u, ' ');
+  a.append("X world!" + 2u, 5u);
+  a.append("X world!" + 7u);
+  EXPECT_STREQ("hello world!", a.c_str());
+}
+
+TEST(FixedStringCAppendTest, CAppendString) {
+  constexpr folly::FixedString<10> a{"hello"}, b{"X world!"};
+  constexpr auto tmp1 = a.cappend(' ');
+  constexpr auto tmp2 = tmp1.cappend(b, 2u, 5u);
+  constexpr auto tmp3 = tmp2.cappend(b, 7u, 1u);
+  static_assert(tmp3 == "hello world!", "");
+}
+
+TEST(FixedStringCAppendTest, CAppendLiteral) {
+  constexpr folly::FixedString<10> a{"hello"};
+  constexpr auto tmp1 = a.cappend(' ');
+  constexpr auto tmp2 = tmp1.cappend("X world!", 2u, 5u);
+  constexpr auto tmp3 = tmp2.cappend("X world!", 7u, 1u);
+  static_assert(tmp3 == "hello world!", "");
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr folly::FixedString<10> constexpr_replace_string_test() {
+  folly::FixedString<10> tmp{"abcdefghij"};
+  tmp.replace(1, 5, FS("XX"));
+  return tmp;
+}
+
+TEST(FixedStringReplaceTest, ConstexprReplaceString) {
+  static_assert(constexpr_replace_string_test().size() == 7u, "");
+  static_assert(constexpr_replace_string_test() == "aXXghij", "");
+}
+#endif
+
+TEST(FixedStringReplaceTest, RuntimeReplaceString) {
+  folly::FixedString<10> tmp{"abcdefghij"};
+  tmp.replace(1, 5, FS("XX"));
+  EXPECT_EQ(7u, tmp.size());
+  EXPECT_STREQ("aXXghij", tmp.c_str());
+}
+
+TEST(FixedStringEraseTest, RuntimeEraseTest) {
+  auto x = FS("abcdefghijklmnopqrstuvwxyz"), y = x;
+  x.erase(x.size());
+  EXPECT_EQ(26u, x.size());
+  EXPECT_STREQ(y.c_str(), x.c_str());
+  x.erase(25u).erase(24u);
+  EXPECT_EQ(24u, x.size());
+  EXPECT_STREQ("abcdefghijklmnopqrstuvwx", x.c_str());
+  x.erase(1u, x.size() - 2u);
+  EXPECT_EQ(2u, x.size());
+  EXPECT_STREQ("ax", x.c_str());
+}
+
+TEST(FixedStringEraseTest, CEraseTest) {
+  constexpr auto x = FS("abcdefghijklmnopqrstuvwxyz"), y = x;
+  constexpr auto tmp0 = x.cerase(x.size());
+  static_assert(26u == tmp0.size(), "");
+  static_assert(y == tmp0, "");
+  constexpr auto tmp1 = tmp0.cerase(25u).cerase(24u);
+  static_assert(24u == tmp1.size(), "");
+  static_assert("abcdefghijklmnopqrstuvwx" == tmp1, "");
+  constexpr auto tmp2 = tmp1.cerase(1u, tmp1.size() - 2u);
+  static_assert(2u == tmp2.size(), "");
+  static_assert("ax" == tmp2, "");
+  constexpr auto tmp3 = tmp2.cerase();
+  static_assert("" == tmp3, "");
+}
+
+TEST(FixedStringFindTest, FindString) {
+  constexpr folly::FixedString<10> tmp{"hijdefghij"};
+  static_assert(tmp.find(FS("hij")) == 0u, "");
+  static_assert(tmp.find(FS("hij"), 1u) == 7u, "");
+  static_assert(tmp.find(FS("hijdefghij")) == 0u, "");
+  static_assert(tmp.find(FS("")) == 0u, "");
+}
+
+TEST(FixedStringFindTest, FindLiteral) {
+  constexpr folly::FixedString<10> tmp{"hijdefghij"};
+  static_assert(tmp.find("hij") == 0u, "");
+  static_assert(tmp.find("hij", 1u) == 7u, "");
+  static_assert(tmp.find("hijdefghij") == 0u, "");
+}
+
+TEST(FixedStringReverseFindTest, FindChar) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find('s') == 3u, "");
+  static_assert(tmp.find('s', 9u) == 10u, "");
+  static_assert(tmp.find('s', 10u) == 10u, "");
+  static_assert(tmp.find('s', 11u) == tmp.npos, "");
+}
+
+TEST(FixedStringReverseFindTest, ReverseFindString) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.rfind(FS("is")) == 5u, "");
+  static_assert(tmp.rfind(FS("is"), 4u) == 2u, "");
+  static_assert(tmp.rfind(FS("This is a string")) == 0u, "");
+  static_assert(tmp.rfind(FS("This is a string!")) == tmp.npos, "");
+  static_assert(tmp.rfind(FS("")) == 16u, "");
+}
+
+TEST(FixedStringReverseFindTest, ReverseFindLiteral) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.rfind("is") == 5u, "");
+  static_assert(tmp.rfind("is", 4u) == 2u, "");
+  static_assert(tmp.rfind("This is a string") == 0u, "");
+  static_assert(tmp.rfind("This is a string!") == tmp.npos, "");
+  static_assert(tmp.rfind("") == 16u, "");
+}
+
+TEST(FixedStringReverseFindTest, ReverseFindChar) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.rfind('s') == 10u, "");
+  static_assert(tmp.rfind('s', 5u) == 3u, "");
+  static_assert(tmp.rfind('s', 3u) == 3u, "");
+  static_assert(tmp.rfind('s', 2u) == tmp.npos, "");
+}
+
+TEST(FixedStringFindFirstOfTest, FindFirstOfString) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_first_of(FS("hi")) == 1u, "");
+  static_assert(tmp.find_first_of(FS("xi")) == 2u, "");
+  static_assert(tmp.find_first_of(FS("xi"), 6u) == 13u, "");
+  static_assert(tmp.find_first_of(FS("xz")) == tmp.npos, "");
+  static_assert(FS("a").find_first_of(FS("cba")) == 0u, "");
+  static_assert(FS("").find_first_of(FS("cba")) == tmp.npos, "");
+  static_assert(FS("a").find_first_of(FS("")) == tmp.npos, "");
+  static_assert(FS("").find_first_of(FS("")) == tmp.npos, "");
+}
+
+TEST(FixedStringFindFirstOfTest, FindFirstOfLiteral) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_first_of("hi") == 1u, "");
+  static_assert(tmp.find_first_of("xi") == 2u, "");
+  static_assert(tmp.find_first_of("xi", 6u) == 13u, "");
+  static_assert(tmp.find_first_of("xis", 6u, 2u) == 13u, "");
+  static_assert(tmp.find_first_of("xz") == tmp.npos, "");
+  static_assert(FS("a").find_first_of("cba") == 0u, "");
+  static_assert(FS("").find_first_of("cba") == tmp.npos, "");
+  static_assert(FS("a").find_first_of("") == tmp.npos, "");
+  static_assert(FS("").find_first_of("") == tmp.npos, "");
+}
+
+TEST(FixedStringFindFirstOfTest, FindFirstOfChar) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_first_of('h') == 1u, "");
+  static_assert(tmp.find_first_of('i') == 2u, "");
+  static_assert(tmp.find_first_of('i', 6u) == 13u, "");
+  static_assert(tmp.find_first_of('x') == tmp.npos, "");
+  static_assert(FS("a").find_first_of('a') == 0u, "");
+  static_assert(FS("").find_first_of('a') == tmp.npos, "");
+}
+
+TEST(FixedStringFindFirstNotOfTest, FindFirstNotOfString) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_first_not_of(FS("Ti")) == 1u, "");
+  static_assert(tmp.find_first_not_of(FS("hT")) == 2u, "");
+  static_assert(tmp.find_first_not_of(FS("s atr"), 6u) == 13u, "");
+  static_assert(tmp.find_first_not_of(FS("This atrng")) == tmp.npos, "");
+  static_assert(FS("a").find_first_not_of(FS("X")) == 0u, "");
+  static_assert(FS("").find_first_not_of(FS("cba")) == tmp.npos, "");
+  static_assert(FS("a").find_first_not_of(FS("")) == 0u, "");
+  static_assert(FS("").find_first_not_of(FS("")) == tmp.npos, "");
+}
+
+TEST(FixedStringFindFirstNotOfTest, FindFirstNotOfLiteral) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_first_not_of("Ti") == 1u, "");
+  static_assert(tmp.find_first_not_of("hT") == 2u, "");
+  static_assert(tmp.find_first_not_of("s atr", 6u) == 13u, "");
+  static_assert(tmp.find_first_not_of("This atrng") == tmp.npos, "");
+  static_assert(FS("a").find_first_not_of("X") == 0u, "");
+  static_assert(FS("").find_first_not_of("cba") == tmp.npos, "");
+  static_assert(FS("a").find_first_not_of("") == 0u, "");
+  static_assert(FS("").find_first_not_of("") == tmp.npos, "");
+}
+
+TEST(FixedStringFindFirstNotOfTest, FindFirstNotOfChar) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_first_not_of('T') == 1u, "");
+  static_assert(tmp.find_first_not_of('i') == 0u, "");
+  static_assert(tmp.find_first_not_of('x', 6u) == 6u, "");
+  static_assert(tmp.find_first_not_of('s', 6u) == 7u, "");
+  static_assert(FS("a").find_first_not_of('a') == tmp.npos, "");
+  static_assert(FS("").find_first_not_of('a') == tmp.npos, "");
+}
+
+TEST(FixedStringFindLastOfTest, FindLastOfString) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_last_of(FS("hi")) == 13u, "");
+  static_assert(tmp.find_last_of(FS("xh")) == 1u, "");
+  static_assert(tmp.find_last_of(FS("xi"), 6u) == 5u, "");
+  static_assert(tmp.find_last_of(FS("xz")) == tmp.npos, "");
+  static_assert(FS("a").find_last_of(FS("cba")) == 0u, "");
+  static_assert(FS("").find_last_of(FS("cba")) == tmp.npos, "");
+  static_assert(FS("a").find_last_of(FS("")) == tmp.npos, "");
+  static_assert(FS("").find_last_of(FS("")) == tmp.npos, "");
+}
+
+TEST(FixedStringFindLastOfTest, FindLastOfLiteral) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_last_of("hi") == 13u, "");
+  static_assert(tmp.find_last_of("xh") == 1u, "");
+  static_assert(tmp.find_last_of("xi", 6u) == 5u, "");
+  static_assert(tmp.find_last_of("xis", 6u, 2u) == 5u, "");
+  static_assert(tmp.find_last_of("xz") == tmp.npos, "");
+  static_assert(FS("a").find_last_of("cba") == 0u, "");
+  static_assert(FS("").find_last_of("cba") == tmp.npos, "");
+  static_assert(FS("a").find_last_of("") == tmp.npos, "");
+  static_assert(FS("").find_last_of("") == tmp.npos, "");
+}
+
+TEST(FixedStringFindLastOfTest, FindLastOfChar) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_last_of('h') == 1u, "");
+  static_assert(tmp.find_last_of('i') == 13u, "");
+  static_assert(tmp.find_last_of('i', 6u) == 5u, "");
+  static_assert(tmp.find_last_of('x') == tmp.npos, "");
+  static_assert(FS("a").find_last_of('a') == 0u, "");
+  static_assert(FS("").find_last_of('a') == tmp.npos, "");
+}
+
+TEST(FixedStringFindLastNotOfTest, FindLastNotOfString) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_last_not_of(FS("gstrin")) == 9u, "");
+  static_assert(tmp.find_last_not_of(FS("hT")) == 15u, "");
+  static_assert(tmp.find_last_not_of(FS("s atr"), 6u) == 5u, "");
+  static_assert(tmp.find_last_not_of(FS("This atrng")) == tmp.npos, "");
+  static_assert(FS("a").find_last_not_of(FS("X")) == 0u, "");
+  static_assert(FS("").find_last_not_of(FS("cba")) == tmp.npos, "");
+  static_assert(FS("a").find_last_not_of(FS("")) == 0u, "");
+  static_assert(FS("").find_last_not_of(FS("")) == tmp.npos, "");
+}
+
+TEST(FixedStringFindLastNotOfTest, FindLastNotOfLiteral) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_last_not_of("gstrin") == 9u, "");
+  static_assert(tmp.find_last_not_of("hT") == 15u, "");
+  static_assert(tmp.find_last_not_of("s atr", 6u) == 5u, "");
+  static_assert(tmp.find_last_not_of(" atrs", 6u, 4u) == 6u, "");
+  static_assert(tmp.find_last_not_of("This atrng") == tmp.npos, "");
+  static_assert(FS("a").find_last_not_of("X") == 0u, "");
+  static_assert(FS("").find_last_not_of("cba") == tmp.npos, "");
+  static_assert(FS("a").find_last_not_of("") == 0u, "");
+  static_assert(FS("").find_last_not_of("") == tmp.npos, "");
+}
+
+TEST(FixedStringFindLastNotOfTest, FindLastNotOfChar) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  static_assert(tmp.find_last_not_of('g') == 14u, "");
+  static_assert(tmp.find_last_not_of('i') == 15u, "");
+  static_assert(tmp.find_last_not_of('x', 6u) == 6u, "");
+  static_assert(tmp.find_last_not_of('s', 6u) == 5u, "");
+  static_assert(FS("a").find_last_not_of('a') == tmp.npos, "");
+  static_assert(FS("").find_last_not_of('a') == tmp.npos, "");
+}
+
+TEST(FixedStringConversionTest, ConversionToStdString) {
+  constexpr folly::FixedString<16> tmp{"This is a string"};
+  std::string str = tmp;
+  EXPECT_STREQ("This is a string", str.c_str());
+  str = "another string"_fs16;
+  EXPECT_STREQ("another string", str.c_str());
+}
+
+#if FOLLY_USE_CPP14_CONSTEXPR
+constexpr std::size_t countSpacesReverse(folly::FixedString<50> s) {
+  std::size_t count = 0u;
+  auto i = s.rbegin();
+  for( ; i != s.rend(); ++i, --i, i++, i--, i+=1, i-=1, i+=1 ) {
+    if (' ' == *i)
+      ++count;
+  }
+  return count;
+}
+
+TEST(FixedStringReverseIteratorTest, Cpp14ConstexprReverseIteration) {
+  static_assert(3 == countSpacesReverse("This is a string"), "");
+}
+#endif
+
+TEST(FixedStringReverseIteratorTest, ConstexprReverseIteration) {
+  static constexpr auto alpha = FS("abcdefghijklmnopqrstuvwxyz");
+  static_assert('a' == alpha.rbegin()[25], "");
+  static_assert('a' == *(alpha.rbegin() + 25), "");
+  static_assert('c' == *(alpha.rbegin() + 25 - 2), "");
+  static_assert((alpha.rend() - 2) == (alpha.rbegin() + 24), "");
+}
+
+#include <folly/Range.h>
+
+TEST(FixedStringConversionTest, ConversionToFollyRange) {
+  // The following declaraction is static for compilers that haven't implemented
+  // the resolution of:
+  // http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1454
+  static constexpr folly::FixedString<16> tmp{"This is a string"};
+  constexpr folly::StringPiece piece = tmp;
+  static_assert(tmp.begin() == piece.begin(), "");
+  static_assert(tmp.end() == piece.end(), "");
+}