[BUG]: libcu++ should support the three-way comparison operator

[ericniebler]

Is this a duplicate?

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Area

libcu++

Is your feature request related to a problem? Please describe.

in cuda/std/detail/libcxx/include/__config there is this:

// FIXME: Correct this macro when either (A) a feature test macro for the
// spaceship operator is provided, or (B) a compiler provides a complete
// implementation.
#  define _LIBCUDACXX_HAS_NO_SPACESHIP_OPERATOR

a lot of code is hidden behind the _LIBCUDACXX_HAS_NO_SPACESHIP_OPERATOR macro, which is set unconditionally. this needs attention.

Describe the solution you'd like

properly support the three-way comparison operator when it is available.

Describe alternatives you've considered

No response

Additional context

No response

[miscco]

This does not work because three-way-comparison operators default to named types within namespace std that are provided by the standard library and are not annotated as __host__ __device__. We do not want to require our users to build with --relaxed-constexpr

We would need to get every standard library on board to add related annotations to the orderings and would most likely need approval to maintain that indefinitely