prog.cc:6:5: error: no matching function for call to 'f'
f<v<int>>(X<0>());
^~~~~~~~~
prog.cc:2:32: note: candidate function not viable: no known conversion from 'X<...>' to 'X<...>' for 1st argument
template<int n> constexpr void f(X<n>) {}
^
1 error generated.