[committed] libstdc++: Apply small fix from LWG 3843 to std::expected

[Jonathan Wakely via Gcc-patches]

Tested powerpc64le-linux. Pushed to trunk.

-- >8 --

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.
---
 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&&
-- 
2.39.2