为什么这段代码用gcc编译而不用clang编译

Question

This code works perfectly fine with gcc/g++ and msvc but not with clang. It keeps complaining that no matching function for Log was found, what is going on?

#include <iostream>

template <typename Function, typename... Args>
auto Call(Function func, Args&&... args) -> typename std::result_of<Function&(Args&&...)>::type
{
    return func(std::forward<Args>(args)...);
}

template <typename T, typename... Args>
T (*Log( T (*FuncPtr)(Args...) ))(Args...)
{
    return FuncPtr;
}

int main()
{
    auto r = Log(Call<int(int), int>)([](int x){
        return x*10;
    }, 10);
    std::cerr << r << std::endl;
}

Error:

> error: no matching function for call to 'Log'
>     auto r = Log(Call<int(int), int>)([](int x){
>              ^~~ test7.cpp:15:5: note: candidate template ignored: couldn't infer template argument 'T' T (*Log( T (*FuncPtr)(Args...)
> ))(Args...)
>     ^ 1 error generated.

Answer 1

I believe that this code is incorrect. The function parameter to Log cannot be used for template argument deduction in this case because the argument is a non-deduced context.

From [temp.deduct.type] in the standard, p5 lists the non-deduced contexts, and p5.5 says:

A function parameter for which argument deduction cannot be done because the associated function argument is a function, or a set of overloaded functions (13.4), and one or more of the following apply:

and p5.5.3 says:

the set of functions supplied as an argument contains one or more function templates.

My interpretation is that you have a function parameter for which the function argument is a (pointer to) a function and that function is a function template.

Arguably, because this isn't an overload set, this might be something that could be allowed in the future, but I read the standard as not guaranteeing that this technique will work.