[gcc r13-5661] libstdc++: Use emplace in std::variant::operator=(T&&) as per LWG 3585

[gcc r13-5661] libstdc++: Use emplace in std::variant::operator=(T&&) as per LWG 3585

[Jonathan Wakely]

https://gcc.gnu.org/g:1395415fdc2d60e5346dbcf476749daf42d5b724

commit r13-5661-g1395415fdc2d60e5346dbcf476749daf42d5b724
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Feb 1 20:54:22 2023 +0000

    libstdc++: Use emplace in std::variant::operator=(T&&) as per LWG 3585
    
    This was approved at the October 2021 plenary.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/variant (variant::operator=): Implement resolution
            of LWG 3585.
            * testsuite/20_util/variant/lwg3585.cc: New test.

Diff:
---
 libstdc++-v3/include/std/variant                  |  4 +++-
 libstdc++-v3/testsuite/20_util/variant/lwg3585.cc | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 35781495e31..5155124522f 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1481,7 +1481,9 @@ namespace __variant
 			    || !is_nothrow_move_constructible_v<_Tj>)
 		this->emplace<__index>(std::forward<_Tp>(__rhs));
 	      else
-		operator=(variant(std::forward<_Tp>(__rhs)));
+		// _GLIBCXX_RESOLVE_LIB_DEFECTS
+		// 3585. converting assignment with immovable alternative
+		this->emplace<__index>(_Tj(std::forward<_Tp>(__rhs)));
 	    }
 	  return *this;
 	}
diff --git a/libstdc++-v3/testsuite/20_util/variant/lwg3585.cc b/libstdc++-v3/testsuite/20_util/variant/lwg3585.cc
new file mode 100644
index 00000000000..0cbfc0db7f5
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/variant/lwg3585.cc
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++17 } }
+
+// LWG 3585. Variant converting assignment with immovable alternative
+
+#include <variant>
+#include <string>
+
+struct A {
+  A() = default;
+  A(A&&) = delete;
+};
+
+int main() {
+  std::variant<A, std::string> v;
+  v = "hello";
+}