Is this program ill-formed despite SFINAE?

Question

template  void f() {
    return 0;  // returning value from function returning `void`
}

int main()
{
    // Not instantiating or calling any f

        

Answer 1

Reading more closely, that standard passage says:

If a substitution results in an invalid type or expression, type deduction fails. An invalid type or expression is one that would be ill-formed if written using the substituted arguments. [..]

return 0 is not an expression, so SFINAE does not apply.

The passage goes on:

Only invalid types and expressions in the immediate context of the function type and its template parameter types can result in a deduction failure.

return 0 has nothing to do with the function type or its template parameter types, so SFINAE still does not apply.