在 C++20 中是否允许通过非类型模板参数中的类类型传递函数指针？

Question

Recently, after playing around with the C++20 feature of being able to pass class types in non-type template parameters ( P0732R2 ) , I've encountered something rather strange.

Consider the following code:

template <typename T>
struct abc {
    T p;
    consteval abc(T p) : p(p) { }
    consteval operator decltype(p)() const {
        return p;
    }
};

template <abc obj>
consteval auto do_something1() {
    return obj();
}

template <auto obj>
consteval auto do_something2() {
    return obj();
}

constexpr auto fun() {
    return true;
}

int main() {
    static_assert(do_something1<&fun>()); // OK in Clang, fails in GCC
    static_assert(do_something2<&fun>()); // OK in both GCC and Clang
}

This appears to work as intended with clang however GCC fails to compile this and gives an error saying that fun is not a valid template argument .

However, this answer says that function pointers should be valid template arguments as per the C++ standard.

Which compiler is correct here?

Also, weirdly enough, the same code appears to work on both compilers when modified to work with member functions for some reason .

Note : Something that might be useful to note is that according to this page, GCC's support for P0732R2 is complete while Clang's is still in a partial/experimental stage. So, does this mean that GCC is correct and passing of function pointers through class types in non-type template parameters shouldn't be possible, or am I missing something here?

Answer 1

Which compiler is correct here?

Clang is correct, the code as written is valid.

GCC not handling that specific case is a known issue .