Proposed resolution:

This wording is relative to N4640.

1. Modify [func.invoke] as indicated:

   template <class F, class... Args>
        constexpr result_of_t<F&&(Args&&...)> invoke(F&& f, Args&&... args);
   

[ 2020-01 Resolved by the adoption of P1065 in Cologne. ]

[ 2017-07 Toronto Tuesday PM issue prioritization ]

Priority 3

[ Kona 2017-03-01 ]

We think this needs CWG 1581 to work; accepted as Immediate to resolve NB comment.

Friday: CWG 1581 was not moved in Kona. Status back to Open.

[ 2017-02-20, Marshall adds wording ]

The function template std::apply() in [tuple.apply] is required to be constexpr, but std::invoke() in [func.invoke] isn't. The most sensible implementation of apply_impl() is exactly equivalent to std::invoke(), so this requires implementations to have a constexpr version of invoke() for internal use, and the public API std::invoke, which must not be constexpr even though it is probably implemented in terms of the internal version.

Proposed change: Add constexpr to std::invoke.