[PATCH] libstdc++: Fix allocator propagation for rvalue+rvalue string concatenation

[Jonathan Wakely]

I made a last-minute change to Nina's r10-200-gf4e678ef74b272
implementation of P1165R1 (consistent allocator propagation for
operator+ on strings), so that the rvalue+rvalue case assumes that COW
strings do not support stateful allocators. I don't think that was true
when the change went in, and certainly isn't true now. COW strings don't
support allocator propagation on assignment and swap, but they do
support non-equal stateful allocators, which are correctly propagated on
move construction.

This removes the preprocessor conditional in the rvalue+rvalue overload
so that COW strings are handled equivalently. Also use constexpr-if
unconditionally, disabling diagnostics with pragmas.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (operator+(string&&, string&&)):
	Do not assume that COW strings have equal allocators. Use
	constexpr-if unconditionally.
	* testsuite/21_strings/basic_string/allocator/char/operator_plus.cc:
	Remove { dg-require-effective-target cxx11_abi }.
	* testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc:
	Likewise.
---

Tested x86_64-linux.

 libstdc++-v3/include/bits/basic_string.h               | 10 ++++++----
 .../basic_string/allocator/char/operator_plus.cc       |  2 --
 .../basic_string/allocator/wchar_t/operator_plus.cc    |  2 --
 3 files changed, 6 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index c90bd099b63..a087e637805 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -3938,21 +3938,23 @@ _GLIBCXX_END_NAMESPACE_CXX11
     operator+(basic_string<_CharT, _Traits, _Alloc>&& __lhs,
 	      basic_string<_CharT, _Traits, _Alloc>&& __rhs)
     {
-#if _GLIBCXX_USE_CXX11_ABI
-      using _Alloc_traits = allocator_traits<_Alloc>;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
+      // Return value must use __lhs.get_allocator(), but if __rhs has equal
+      // allocator then we can choose which parameter to modify in-place.
       bool __use_rhs = false;
-      if _GLIBCXX17_CONSTEXPR (typename _Alloc_traits::is_always_equal{})
+      if constexpr (allocator_traits<_Alloc>::is_always_equal::value)
 	__use_rhs = true;
       else if (__lhs.get_allocator() == __rhs.get_allocator())
 	__use_rhs = true;
       if (__use_rhs)
-#endif
 	{
 	  const auto __size = __lhs.size() + __rhs.size();
 	  if (__size > __lhs.capacity() && __size <= __rhs.capacity())
 	    return std::move(__rhs.insert(0, __lhs));
 	}
       return std::move(__lhs.append(__rhs));
+#pragma GCC diagnostic pop
     }
 
   template<typename _CharT, typename _Traits, typename _Alloc>
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/operator_plus.cc b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/operator_plus.cc
index 571f8535176..92e05690b19 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/operator_plus.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/char/operator_plus.cc
@@ -17,8 +17,6 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-do run { target c++11 } }
-// COW strings don't support C++11 allocator propagation:
-// { dg-require-effective-target cxx11_abi }
 
 #include <string>
 #include <testsuite_hooks.h>
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc
index 0da684360ab..b75b26ae85c 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc
@@ -17,8 +17,6 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-do run { target c++11 } }
-// COW strings don't support C++11 allocator propagation:
-// { dg-require-effective-target cxx11_abi }
 
 #include <string>
 #include <testsuite_hooks.h>
-- 
2.49.0