Fix broken clause11_21_4_6_6 test in Apple Clang.

Summary:
- Prior to this fix, the test relied upon begin() being evaluated after
the fbstring constructor (even thoug the order of evaluation of
function arguments is un-defined). When this assumption was violated,
begin() ended up with an iterator that was invalid since it points to
data internal to fbstring, and the fbstring copy constructor can end
up triggering re-allocation in the source.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1014093

diff --git a/folly/test/FBStringTest.cpp b/folly/test/FBStringTest.cpp
index 3efd652e4a9647ec08009ddf1318ada672777070..4f0aa89af1e5fb9f78f727f5228ca0e0df360be7 100644 (file)
--- a/folly/test/FBStringTest.cpp
+++ b/folly/test/FBStringTest.cpp
@@ -526,10 +526,11 @@ template <class String> void clause11_21_4_6_6(String & test) {
                random(0, maxString), random('a', 'z'));
   pos = random(0, test.size());
   if (avoidAliasing) {
+    auto newString = String(test);
     test.replace(
       test.begin() + pos,
       test.begin() + pos + random(0, test.size() - pos),
-      String(test));
+      newString);
   } else {
     test.replace(
       test.begin() + pos,
@@ -538,10 +539,11 @@ template <class String> void clause11_21_4_6_6(String & test) {
   }
   pos = random(0, test.size());
   if (avoidAliasing) {
+    auto newString = String(test);
     test.replace(
       test.begin() + pos,
       test.begin() + pos + random(0, test.size() - pos),
-      String(test).c_str(),
+      newString.c_str(),
       test.size() - random(0, test.size()));
   } else {
     test.replace(