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.

(cherry picked from commit ce39714a1ce58f2f32e8a44a224061290670db0f)

diff --git a/libstdc++-v3/include/std/expected b/libstdc++-v3/include/std/expected
index 3b97b7b59d859013bd983bc6a65fcf1a74623d34..ed123fb9b594784be4b2a465f2129714aebcc314 100644 (file)
--- a/libstdc++-v3/include/std/expected
+++ b/libstdc++-v3/include/std/expected
@@ -714,7 +714,8 @@ 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&&