[committed] libstdc++: Fix constraint on std::optional assignment [PR 100982]

[Jonathan Wakely via Gcc-patches]

libstdc++-v3/ChangeLog:

        PR libstdc++/100982
        * include/std/optional (optional::operator=(const optional<U>&)):
        Fix value category used in is_assignable check.
        * testsuite/20_util/optional/assignment/100982.cc: New test.

Tested powerpc64le-linux. Committed to trunk.


commit b3fce1bd45f72cc9e55fb7431762e92e30fefcf1
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Jun 9 11:03:15 2021

    libstdc++: Fix constraint on std::optional assignment [PR 100982]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/100982
            * include/std/optional (optional::operator=(const optional<U>&)):
            Fix value category used in is_assignable check.
            * testsuite/20_util/optional/assignment/100982.cc: New test.

diff --git a/libstdc++-v3/include/std/optional 
b/libstdc++-v3/include/std/optional
index 415f8c49ef4..0a67ce24bbd 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -815,7 +815,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Up>
        enable_if_t<__and_v<__not_<is_same<_Tp, _Up>>,
                            is_constructible<_Tp, const _Up&>,
-                           is_assignable<_Tp&, _Up>,
+                           is_assignable<_Tp&, const _Up&>,
                            __not_<__converts_from_optional<_Tp, _Up>>,
                            __not_<__assigns_from_optional<_Tp, _Up>>>,
                    optional&>
diff --git a/libstdc++-v3/testsuite/20_util/optional/assignment/100982.cc 
b/libstdc++-v3/testsuite/20_util/optional/assignment/100982.cc
new file mode 100644
index 00000000000..ae565250d68
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/optional/assignment/100982.cc
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++17 } }
+
+#include <optional>
+
+struct U {};
+
+struct T {
+  explicit T(const U&);
+  T& operator=(const U&);
+  T& operator=(U&&) = delete;
+};
+
+int main() {
+  std::optional<U> opt1;
+  std::optional<T> opt2;
+  opt2 = opt1; // PR libstdc++/100982
+}