Detect mismatch of execution spaces in functors

[masterleinad]

Fixes #2074.

[ldh4]

I was in a process of converting my initial solution, similar to this, into a runtime check in attempt to make an unit-test, but I think compile-time check is sufficient, given that it is a fairly simple check.

[dalg24]

I would prefer if you used is_detected<execution_space_t, Policy>::value instead of std::is_same<detected_t<execution_space_t, Policy>, nonesuch>::value

[masterleinad]

@dalg24 I addressed your suggestions.

In my opinion, the changes here would be a good example of why defining detected_or would be convenient. I could use the result both for checking that the check was successful and get the referenced type in case of success.

[dalg24]

In my opinion, the changes here would be a good example of why defining detected_or would be convenient. I could use the result both for checking that the check was successful and get the referenced type in case of success.

Fair enough. I still don't like the idea of writing

using FunctorExecutionSpace = detected_or<MyDefault, execution_space_t, Functor>;
static_assert(!typename FunctorExecutionSpace::value_type::value || ...);
// or
static_assert(!std::is_same<MyDefault, typename FunctorExecutionSpace::type>::value || ...);