我们如何使用模板参数类型的变量？

Question

In the code below, I passed both class type R and a constant expression of that type as template parameters. But clang doesn't accept that:

#include <iostream>

template<class T, T t>
void foo(){ std::cout << "foo()" << std::endl; }

class R
{
public:
    int f;
    constexpr R(): f(15){ }
};

constexpr R r;

int main(){ foo<R, r>(); } //note: candidate template ignored: 
                           //invalid explicitly-specified argument 
                           //for template parameter 't'

DEMO

In N4296::14.3.2 [temp.arg.nontype] I can't find any restriction other than the non-type template parameter should be a constant expression.

Answer 1

§14.1 [temp.param]/p4:

A non-type template-parameter shall have one of the following (optionally cv-qualified) types:

• integral or enumeration type,
• pointer to object or pointer to function,
• lvalue reference to object or lvalue reference to function,
• pointer to member,
• std::nullptr_t .

A class type is none of these.