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.

(cherry picked from commit 1395415fdc2d60e5346dbcf476749daf42d5b724)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index df0dd08d90a2885346c597cede5483071097c4fe..2e784c744d3eb5409b102a59a8b4503bbc873487 100644 (file)
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1480,7 +1480,9 @@ 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 (file)
index 0000000..0cbfc0d
--- /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";
+}