[gcc r13-6935] libstdc++: Apply small fix from LWG 3843 to std::expected

[Jonathan Wakely]

https://gcc.gnu.org/g:ce39714a1ce58f2f32e8a44a224061290670db0f

commit r13-6935-gce39714a1ce58f2f32e8a44a224061290670db0f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Mar 29 22:43:16 2023 +0100

    libstdc++: Apply small fix from LWG 3843 to std::expected
    
    LWG 3843 adds some type requirements to std::expected::value to ensure
    that it can correctly copy the error value if it needs to throw an
    exception. We don't need to do anything to enforce that, because it will
    already be ill-formed if the type can't be copied. The issue also makes
    a small drive-by fix to ensure that a const E& is copied from the
    non-const value()& overload, which this change implements.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/expected (expected::value() &): Use const lvalue
            for unex member passed to bad_expected_access constructor, as
            per LWG 3843.

Diff:
---
 libstdc++-v3/include/std/expected | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/expected b/libstdc++-v3/include/std/expected
index cb5754e2a68..058188248bb 100644
--- a/libstdc++-v3/include/std/expected
+++ b/libstdc++-v3/include/std/expected
@@ -736,7 +736,8 @@ namespace __expected
       {
 	if (_M_has_value) [[likely]]
 	  return _M_val;
-	_GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(_M_unex));
+	const auto& __unex = _M_unex;
+	_GLIBCXX_THROW_OR_ABORT(bad_expected_access<_Er>(__unex));
       }
 
       constexpr const _Tp&&