How can we use a variable of a type of a template parameter?

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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.

asked 45 secs ago

Dmitry Fucintv

3,873

How can we use a variable of a type of a template parameter?