portability macro for checking whether an expression satisfies a concept in a _CCCL_REQUIRES_EXPR clause

[ericniebler]

Description

when writing a concept definition in CCCL today, if you want to portably test whether a particular expression satisfies a given concept, you have to write it like:

_CCCL_REQUIRES_EXPR((T), T& t)
(
  requires(my_concept<decltype(some-expression)>)
)

in c++20, this expands to:

requires(T& t)
{
  requires my_concept<decltype(some-expression)>;
}

but that is not how you would write it in c++20, and the above formulation gives worse diagnostics than the more idiomatic:

requires(T& t)
{
  {some-expression} -> my_concept;
}

this pr adds a new _Satisfies requirement kind to CCCL's concept portability macros. it would be used like this:

_CCCL_REQUIRES_EXPR((T), T& t)
(
  _Satisfies(my_concept) some-expression
)

in c++20, this expands to the more idiomatic formulation

this pr also changes the equality_comparable and totally_ordered concepts to use _Satisfies.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[ericniebler]

/ok to test a59ca7e

[github-actions]

🥳 CI Workflow Results

🟩 Finished in 3h 14m: Pass: 100%/91 | Total: 2d 18h | Max: 3h 12m | Hits: 61%/208416

See results here.