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.

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;
 	}

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";
+}