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 will prevent this?

Answer 1

Update: P1065 will make it constexpr.

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Keep original post for historical reason:

From the proposal:

Although there is possibility to implement standard conforming invoke function template as a constexpr function, the proposed wording does not require such implementation. The main reason is to left it consistent with existing standard function objects, that could have such definition, like std::mem_fn, std::reference_wrapper and operator wrappers. Furthermore imposing such requirement will block the implementation of invoke that refers to std::mem_fn.

This proposal assumes that constexpr addition to the header would be applied consistently by a separate proposal.

Both constexpr and standard library based implementation are presented in Implementability section of the proposal.

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Related CWG issue #1581: When are constexpr member functions defined?.