[PATCH] Proposed change to the constructor of std::pair

[Marshall Clow]

Hi EricWF, howard.hinnant,

Eric Niebler proposed this as a fix for http://llvm.org/bugs/show_bug.cgi?id=21917.
I've tested it, and found that it does solve the problem in the PR.

Note that this is (probably) a defect in the standard, and that this would be an extension.

My concern is to make sure that this is (a) a conforming extension, and (b) not an ABI change.
The standard says that the default constructor for pair requires that both types be default-constructible.

I don't have tests for this (yet), but the program in the bug now compiles w/o error.

http://reviews.llvm.org/D7384

Files:
  include/utility

Index: include/utility
===================================================================
--- include/utility
+++ include/utility
@@ -260,7 +260,14 @@
     // pair(const pair&) = default;
     // pair(pair&&) = default;
 
-    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR pair() : first(), second() {}
+    template<bool __b = true,
+        typename = typename enable_if<
+               std::is_default_constructible<_T1>::value
+            && std::is_default_constructible<_T2>::value
+            && __b>::type
+        >
+    _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR 
+    pair() : first(), second() {}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX11
     pair(const _T1& __x, const _T2& __y)

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