From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 1 Feb 2023 20:54:22 +0000 (+0000)
Subject: libstdc++: Use emplace in std::variant::operator=(T&&) as per LWG 3585
X-Git-Tag: basepoints/gcc-14~1546
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1395415fdc2d60e5346dbcf476749daf42d5b724;p=thirdparty%2Fgcc.git

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 --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 35781495e311..5155124522f2 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 000000000000..0cbfc0db7f58
--- /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";
+}