Fix warnings about pessimizing return moves for C++11 and higher

Summary:
Throughout the libc++ headers, there are a few instances where
_VSTD::move() is used to return a local variable.  Howard commented in
r189039 that these were there "for non-obvious reasons such as to help
things limp along in C++03 language mode".

However, when compiling these headers with warnings on, and in C++11 or
higher mode (like we do in FreeBSD), they cause the following complaints
about pessimizing moves:

    In file included from tests.cpp:26:
    In file included from tests.hpp:29:
    /usr/include/c++/v1/map:1368:12: error: moving a local object in a return statement prevents copy elision [-Werror,-Wpessimizing-move]
        return _VSTD::move(__h);  // explicitly moved for C++03
               ^
    /usr/include/c++/v1/__config:368:15: note: expanded from macro '_VSTD'
    #define _VSTD std::_LIBCPP_NAMESPACE
                  ^

Attempt to fix this by adding a _LIBCPP_EXPLICIT_MOVE() macro to
__config, which gets defined to _VSTD::move for pre-C++11, and to
nothing for C++11 and later.

I am not completely satisfied with the macro name (I also considered
_LIBCPP_COMPAT_MOVE and some other variants), so suggestions are
welcome. :)

Reviewers: mclow.lists, howard.hinnant, EricWF

Subscribers: arthur.j.odwyer, cfe-commits

Differential Revision: http://reviews.llvm.org/D11394

llvm-svn: 245421
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 251c6291170cf6c999655626a0e246c9f7117dab
diff --git a/include/__config b/include/__config
index 600f0fb..5c15156 100644
--- a/include/__config
+++ b/include/__config
@@ -716,6 +716,12 @@
 #define _LIBCPP_CONSTEXPR_AFTER_CXX11
 #endif
 
+#ifdef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+#  define _LIBCPP_EXPLICIT_MOVE(x) _VSTD::move(x)
+#else
+#  define _LIBCPP_EXPLICIT_MOVE(x) (x)
+#endif
+
 #ifndef _LIBCPP_HAS_NO_ASAN
 extern "C" void __sanitizer_annotate_contiguous_container(
   const void *, const void *, const void *, const void *);
diff --git a/include/__hash_table b/include/__hash_table
index ec20e10..5cfff5a 100644
--- a/include/__hash_table
+++ b/include/__hash_table
@@ -2108,7 +2108,7 @@
     __h.get_deleter().__value_constructed = true;
     __h->__hash_ = hash_function()(__h->__value_);
     __h->__next_ = nullptr;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
@@ -2124,7 +2124,7 @@
     __h.get_deleter().__value_constructed = true;
     __h->__hash_ = __hash;
     __h->__next_ = nullptr;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
diff --git a/include/__tree b/include/__tree
index 3ad5794..6391609 100644
--- a/include/__tree
+++ b/include/__tree
@@ -1843,7 +1843,7 @@
     __node_holder __h(__node_traits::allocate(__na, 1), _Dp(__na));
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_), __v);
     __h.get_deleter().__value_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
diff --git a/include/algorithm b/include/algorithm
index c1635fc..9088829 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -851,7 +851,7 @@
 {
     for (; __first != __last; ++__first)
         __f(*__first);
-    return _VSTD::move(__f);  // explicitly moved for (emulated) C++03
+    return _LIBCPP_EXPLICIT_MOVE(__f);  // explicitly moved for (emulated) C++03
 }
 
 // find
diff --git a/include/ext/hash_map b/include/ext/hash_map
index 31fcedf..0e4ab69 100644
--- a/include/ext/hash_map
+++ b/include/ext/hash_map
@@ -681,7 +681,7 @@
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
diff --git a/include/map b/include/map
index eb6b8ed..561c3dd 100644
--- a/include/map
+++ b/include/map
@@ -1523,7 +1523,7 @@
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Compare, class _Allocator>
diff --git a/include/unordered_map b/include/unordered_map
index cf70ab6..3c774d4 100644
--- a/include/unordered_map
+++ b/include/unordered_map
@@ -1511,7 +1511,7 @@
     __h.get_deleter().__first_constructed = true;
     __node_traits::construct(__na, _VSTD::addressof(__h->__value_.__cc.second));
     __h.get_deleter().__second_constructed = true;
-    return _VSTD::move(__h);  // explicitly moved for C++03
+    return _LIBCPP_EXPLICIT_MOVE(__h);  // explicitly moved for C++03
 }
 
 template <class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>