c++ - How to check if template argument is a callable with a given signature

Question

Basically, what I want to achieve is compile-time verification (with possibly nice error message) that registered callable (either a function, a lambda, a struct with call operator) has correct signature. Example (contents of the static_assert are to be filled):

struct A {
  using Signature = void(int, double);

  template <typename Callable>
  void Register(Callable &&callable) {
    static_assert(/* ... */);
    callback = callable;
  }

  std::function<Signature> callback;
};

Answer 1

You can use std::is_convertible (since C++11), e.g.

static_assert(std::is_convertible_v<Callable&&, std::function<Signature>>, "Wrong Signature!");

or

static_assert(std::is_convertible_v<decltype(callable), decltype(callback)>, "Wrong Signature!");

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