From 2cb63bedc5b9342f2f80491da7e53afd97de1927 Mon Sep 17 00:00:00 2001
From: Elizabeth Smith <elizabeths@fb.com>
Date: Thu, 8 May 2014 08:20:48 -0700
Subject: [PATCH] Expression SFINAE fixes in ApplyTuple

Summary:
MSVC does not support Expression SFINAE

http://stackoverflow.com/questions/12654067

this is a very nice c++11 feature that makes for some nice clean templating

But of course MSVC can't have nice things - it partially implements this when it feels like it, so some will work and some will need the nonsense

@override-unit-failures

There will be more of these little template helper fixes  - they make the code a bit more complex but don't actually change anything when compiled

The accompanying fix in the test also does nothing but work around an MSVC compiler bug where it loses it's mind over the global namespace

Test Plan: fbconfig -r folly && fbmake runtests

Reviewed By: delong.j@fb.com

FB internal diff: D1312700
---
 folly/ApplyTuple.h            | 18 ++++++++++++++----
 folly/test/ApplyTupleTest.cpp |  4 ++++
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/folly/ApplyTuple.h b/folly/ApplyTuple.h
index 3fe30c3d..6fe6093d 100644
--- a/folly/ApplyTuple.h
+++ b/folly/ApplyTuple.h
@@ -51,13 +51,24 @@ struct DerefSize
   : std::tuple_size<typename std::remove_reference<Tuple>::type>
 {};
 
+template<class Tuple, class ...Unpacked> struct ExprDoUnpack {
+  enum {
+    value = sizeof...(Unpacked) < DerefSize<Tuple>::value
+  };
+};
+
+template<class Tuple, class ...Unpacked> struct ExprIsUnpacked {
+  enum {
+    value = sizeof...(Unpacked) == DerefSize<Tuple>::value
+  };
+};
+
 // CallTuple recursively unpacks tuple arguments so we can forward
 // them into the function.
 template<class Ret>
 struct CallTuple {
   template<class F, class Tuple, class ...Unpacked>
-  static typename std::enable_if<
-    (sizeof...(Unpacked) < DerefSize<Tuple>::value),
+  static typename std::enable_if<ExprDoUnpack<Tuple, Unpacked...>::value,
     Ret
   >::type call(const F& f, Tuple&& t, Unpacked&&... unp) {
     typedef typename std::tuple_element<
@@ -71,8 +82,7 @@ struct CallTuple {
   }
 
   template<class F, class Tuple, class ...Unpacked>
-  static typename std::enable_if<
-    (sizeof...(Unpacked) == DerefSize<Tuple>::value),
+  static typename std::enable_if<ExprIsUnpacked<Tuple, Unpacked...>::value,
     Ret
   >::type call(const F& f, Tuple&& t, Unpacked&&... unp) {
     return makeCallable(f)(std::forward<Unpacked>(unp)...);
diff --git a/folly/test/ApplyTupleTest.cpp b/folly/test/ApplyTupleTest.cpp
index 0f831942..7ddac05f 100644
--- a/folly/test/ApplyTupleTest.cpp
+++ b/folly/test/ApplyTupleTest.cpp
@@ -21,6 +21,10 @@
 
 #include <memory>
 
+// this placates visual studio stupidity - see
+// http://stackoverflow.com/questions/5503901
+namespace {}
+
 namespace {
 
 void func(int a, int b, double c) {
-- 
2.34.1