Differences of the interpretation of a non-dependent construct between definition context and point of instantiation in c++

Question

N4527 14.6 [temp.res]/p8

If a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a constru

Answer 1

Here's an example:

extern const int b;

template
void f(int);

template
void f(long);

template
void g() {
    f<0, b>(0);
}
// #1

extern const int b = 0;


int main(){
    g(); 
}

// #2

A hypothetical instantiation at #1 will call void f<0, b>(long), because b isn't a constant expression at that point, so the (int) overload SFINAEs away. An instantiation at #2 (which is a point of instantiation of g) will call void f<0, 0>(int), because by then b is a constant expression, the (int) overload is viable and wins overload resolution.

Clang and GCC will in fact call different fs with this code.