libstdc++: std::string move assignment should not use POCCA trait [PR116641]

The changes to implement LWG 2579 (r10-327-gdb33efde17932f) made
std::string::assign use the propagate_on_container_copy_assignment
(POCCA) trait, for consistency with operator=(const basic_string&).
However, this also unintentionally affected operator=(basic_string&&)
which calls assign(str) to make a deep copy when performing a move is
not possible. The fix is for the move assignment operator to call
_M_assign(str) instead of assign(str), as this just does the deep copy
and doesn't check the POCCA trait first.

The bug only affects the unlikely/useless combination of POCCA==true and
POCMA==false, but we should fix it for correctness anyway. it should
also make move assignment slightly cheaper to compile and execute,
because we skip the extra code in assign(const basic_string&).

libstdc++-v3/ChangeLog:

	PR libstdc++/116641
	* include/bits/basic_string.h (operator=(basic_string&&)): Call
	_M_assign instead of assign.
	* testsuite/21_strings/basic_string/allocator/116641.cc: New
	test.

2 files changed, 54 insertions, 1 deletions

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 944bd23..120c0bc 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -915,7 +915,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 	      __str._M_data(__str._M_use_local_data());
 	  }
 	else // Need to do a deep copy
-	  assign(__str);
+	  _M_assign(__str);
 	__str.clear();
 	return *this;
       }
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/allocator/116641.cc b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/116641.cc
new file mode 100644
index 0000000..a1a411b
--- /dev/null
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/allocator/116641.cc
@@ -0,0 +1,53 @@
+// { dg-do run { target c++11 } }
+// { dg-require-effective-target cxx11_abi }
+
+// Bug 116641 - std::string move assignment incorrectly depends on POCCA
+
+#include <string>
+#include <testsuite_hooks.h>
+
+template<typename T>
+struct Alloc
+{
+  using value_type = T;
+  using propagate_on_container_swap = std::false_type;
+  using propagate_on_container_copy_assignment = std::true_type;
+  using propagate_on_container_move_assignment = std::false_type;
+
+  Alloc(int id) : id(id) { }
+
+  template<typename U>
+    Alloc(const Alloc<U>& a) : id(a.id) { }
+
+  T* allocate(unsigned long n)
+  { return std::allocator<T>().allocate(n); }
+
+  void deallocate(T* p, unsigned long n)
+  { std::allocator<T>().deallocate(p, n); }
+
+  Alloc& operator=(const Alloc&) { throw; }
+
+  bool operator==(const Alloc& a) const { return id == a.id; }
+  bool operator!=(const Alloc& a) const { return id != a.id; }
+
+  int id;
+};
+
+void
+test_pr116641()
+{
+  Alloc<char> a1(1), a2(2);
+  std::basic_string<char, std::char_traits<char>, Alloc<char>> s1(a1), s2(a2);
+
+  s1 = "allocator should not propagate on move assignment";
+  VERIFY( s1.get_allocator() == a1 );
+  VERIFY( s2.get_allocator() == a2 );
+  s2 = std::move(s1);
+  VERIFY( s1.get_allocator() == a1 );
+  VERIFY( s2.get_allocator() == a2 );
+}
+
+int main()
+{
+  test_pr116641();
+}