std :: add_pointer_t <std :: remove_pointer_t <U >>意外行为

Question

为什么以下内容无法编译：

#include <iostream>
#include <cstring>

template <typename U>
void f_broken(std::add_pointer_t<std::remove_pointer_t<U>> p)
{
}

template <typename U>
void f_ok(U p)
{
}


int main() {
    int * ptr;

    std::cout << typeid(ptr).hash_code() << std::endl;
    std::cout << typeid(std::add_pointer_t<std::remove_pointer_t<decltype(ptr)>>).hash_code() << std::endl;

    // f_broken(ptr); Compiler error.
    f_ok(ptr) // Works.
};

类型与预期的相同，但使用时表现不正常。

编译器错误为：

<source>:21:17: error: no matching function for call to 'f_broken(int*&)'

     f_broken(ptr);

                 ^

<source>:5:6: note: candidate: 'template<class U> void f_broken(std::add_pointer_t<typename std::remove_pointer<_Tp>::type>)'

 void f_broken(std::add_pointer_t<std::remove_pointer_t<U>> p)

      ^~~~~~~~

<source>:5:6: note:   template argument deduction/substitution failed:

<source>:21:17: note:   couldn't deduce template parameter 'U'

     f_broken(ptr);

运行示例：https://godbolt.org/z/UUYHvM