__completion_signatures_of_t requires that get_completion_signatures<S,E...>() is a constant expression

[ericniebler]

currently, the definition of the __completion_signatures_of_t alias is:

  using __completion_signatures_of_t =
    decltype(STDEXEC::get_completion_signatures<_Sender, _Env...>());

the trouble with this is that if get_completion_signatures exits with a compile-time exception, that error is lost, and we end up aliasing the declared return type of get_completion_signatures, aka completion_signatures<>.

a better definition looks like

  using __completion_signatures_of_t =
    __mtypeof<STDEXEC::get_completion_signatures<_Sender, _Env...>()>;

where __mtypeof is simply:

template <auto V>
using __mtypeof = decltype(V);

this causes __completion_signatures_of_t to SFINAE when the call to get_completion_signatures is not a constant expression.