Why is `std :: invoke` not constexpr?

Question

Shouldn't std :: invoke be constexpr especially after constexpr lambdas in c ++ 17 ?

Are there any obstacles that could prevent this?

Answer 1

From the offer :

Although it is possible to implement the standard invoke matching pattern as a constexpr function, the proposed formulation does not require such an implementation. The main reason is to leave it in accordance with existing objects of standard functions that may have such a definition, for example std :: mem_fn, std :: reference_wrapper and operator shells. In addition, the imposition of such a requirement blocks the implementation of a call that refers to std :: mem_fn.

This sentence assumes that adding constexpr to the header will be applied sequentially in a separate sentence.

Both options for implementing constexpr and the standard library are presented in the "Realism" section of the proposal.

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