Use of `promise_contract_t<T>::static_query_v<E> == true` is invalid

[chriskohlhoff]

promise_contract_t<T>::static_query_v<E> == true is invalid since promise_contract_t doesn't support static_query_v (i.e. a compile-time constant expression query). Was it supposed to be can_query_v<E, promise_contract_t<T>>?

[dhollman]

@LeeHowes @brycelelbach I can make this change.

[LeeHowes]

Go ahead.

[dhollman]

A couple of problems:

• If via() returns the second half of the promise contract pair directly, how is an executor supposed to implement the promise_contract_t return type with a future token rather than a full-fledged executor_future_t?
• Is it ever reasonable for the query of the promise_contract_t to be constexpr constructible? If not, we should remove static_query_v from the definition of the properties.

[brycelelbach]

Is this fixed?

[dhollman]

Mostly. static_query_v is still there for cancellable_promise_contract. The part in the SemiFuture requirements table also still needs to be fixed. I'll put in a pull request later this week.