commit d83b3f196db2f1f9fdc1da7d9c78c4ad2d512802
author Louis Dionne <ldionne@apple.com> Wed Sep 23 08:49:00 2020 -0400
committer Copybara-Service <copybara-worker@google.com> Thu Oct 29 17:46:52 2020 -0700
tree 7ce7b152f9ed1a3469d7126d2dfb62fa39dcddc1
parent 9e3f3be4fe71d0e0a27531e7a53b091235855e2b

[libc++] Re-apply fdc41e11f (LWG1203) without breaking the C++11 build

fdc41e11f was reverted in e46c1def5 because it broke the C++11 build.
We shouldn't be using enable_if_t in C++11, instead we must use
enable_if<...>::type.

GitOrigin-RevId: c90dee1e90045feb039be640864f038eebd1d8cd

include/ostream

diff --git a/include/ostream b/include/ostream
index 697732d..809f601 100644
--- a/include/ostream
+++ b/include/ostream
@@ -126,9 +126,8 @@
   basic_ostream<charT,traits>& flush(basic_ostream<charT,traits>& os);
 
 // rvalue stream insertion
-template <class charT, class traits, class T>
-  basic_ostream<charT, traits>&
-  operator<<(basic_ostream<charT, traits>&& os, const T& x);
+template <class Stream, class T>
+  Stream&& operator<<(Stream&& os, const T& x);
 
 }  // std
 
@@ -1028,15 +1027,20 @@
 
 #ifndef _LIBCPP_CXX03_LANG
 
+template <class _Stream, class _Tp, class = void>
+struct __is_ostreamable : false_type { };
+
 template <class _Stream, class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY
-typename enable_if
-<
-    !is_lvalue_reference<_Stream>::value &&
-    is_base_of<ios_base, _Stream>::value,
-    _Stream&&
->::type
-operator<<(_Stream&& __os, const _Tp& __x)
+struct __is_ostreamable<_Stream, _Tp, decltype(
+    _VSTD::declval<_Stream>() << _VSTD::declval<_Tp>(), void()
+)> : true_type { };
+
+template <class _Stream, class _Tp, class = typename enable_if<
+    _And<is_base_of<ios_base, _Stream>,
+         __is_ostreamable<_Stream&, const _Tp&>>::value
+>::type>
+_LIBCPP_INLINE_VISIBILITY
+_Stream&& operator<<(_Stream&& __os, const _Tp& __x)
 {
     __os << __x;
     return _VSTD::move(__os);