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

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux. Pushed to trunk.

-- >8 --

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                 |  4 +++-
 .../testsuite/20_util/variant/lwg3585.cc         | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/variant/lwg3585.cc

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";
+}
-- 
2.39.1