[gcc r10-9915] libstdc++: Fix constraint on std::optional assignment [PR 100982]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:37864c5e6aa6befc17051f162c02d4f593a57e57

commit r10-9915-g37864c5e6aa6befc17051f162c02d4f593a57e57
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 9 11:03:15 2021 +0100

    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.
    
    (cherry picked from commit b3fce1bd45f72cc9e55fb7431762e92e30fefcf1)

Diff:
---
 libstdc++-v3/include/std/optional                       |  2 +-
 .../testsuite/20_util/optional/assignment/100982.cc     | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index d473b5369be..e3fbe625f25 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -798,7 +798,7 @@ _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
+}