[committed] libstdc++: Fix constraints on std::optional comparisons [PR 96269]

[Jonathan Wakely]

On 05/11/20 19:09 +0000, Jonathan Wakely wrote:
>The relational operators for std::optional were using the wrong types
>in the declval expressions used to constrain them. Instead of using
>const lvalues they were using non-const rvalues, which meant that a type
>might satisfy the constraints but then give an error when the function
>body was instantiated.
>
>libstdc++-v3/ChangeLog:
>
>	PR libstdc++/96269
>	* include/std/optional (operator==, operator!=, operator<)
>	(operator>, operator<=, operator>=): Fix types used in
>	SFINAE constraints.
>	* testsuite/20_util/optional/relops/96269.cc: New test.
>
>Tested powerpc64le-linux. Committed to trunk.

When concepts are supported we can make the alias templates
__optional_eq_t et al use a requires-expression instead of SFINAE.
This is potentially faster to compile, given expected improvements
to C++20 compilers.

I'm testing this patch.

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