C++20 是否允许通过非类型模板参数中的 class 类型传递函数指针？

Question

最近在玩弄C++20的可以通过class types in non-type template parameters (P0732R2)的特性后，遇到了一些比较奇怪的事情。

考虑以下代码：

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
}

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

但是，this answer 说函数指针应该是符合 C++ 标准的有效模板参数。

这里哪个编译器是正确的？

此外，奇怪的是，相同的代码 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 是正确的，所写的代码是有效的。

GCC 不处理该特定案例是已知的 issue。

C++20 是否允许通过非类型模板参数中的 class 类型传递函数指针？

Is passing of a function pointer through a class type in non-type template parameter allowed in C++20?

c++

function-pointers

language-lawyer

c++20

non-type-template-parameter