noexcept operator fails after calling pointer-to-member function

Question

This MWE may appear contrived, but the failing static_assert is surprising nonetheless:

#include <utility>

struct C {
  void f() noexcept { }
  using F = void(C::*)();

  static constexpr F handler() noexcept {
    return &C::f;
  }

  void g() noexcept(noexcept((this->*handler())())) {
  }
};

int main() {
  static_assert(noexcept(std::declval<C>().g()));
}

Wandbox link: https://wandbox.org/permlink/a8HSyfuyX1buGrbZ

I would expect this to work on Clang but not GCC due to their different treatments of "this" in the context of operator noexcept.

Answer 1

Seeing as your static_assert has no string argument, you are using C++17. In C++17, noexcept became part of the type system. What this means is that given:

using F = void(C::*)();

This PMF is not noexcept. Calling it is equivalent to calling a noexcept(false) member function. You need to mark the function type as noexcept:

using F = void(C::*)() noexcept;

That change allows your code to compile:

#include <utility>

struct C {
  void f() noexcept { }
  using F = void(C::*)() noexcept;

  static constexpr F handler() noexcept {
    return &C::f;
  }

  void g() noexcept(noexcept((this->*handler())())) {
  }
};

int main() {
  static_assert(noexcept(std::declval<C>().g()));
}

^{On Godbolt}

Answer 2

f is noexcept, but the pointer to it is not. So in the definition of g, this->*handler() returns a PMF which is not noexcept (even though you happen to return the address of a MF that is noexcept, so when you call that by writing (this->*handler())() then you are calling a function which is not noexcept, so the noexcept clause there returns false.

Add noexcept to the end of line 5 and it works.