This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

368. Uses of non-type parameters that should cause deduction to fail

[Voted into WP at October 2003 meeting.]

I understand the rules in 13.10.3 [temp.deduct] paragraph 2 are meant to be an exhaustive list of what can cause type deduction to fail.

Consider:

  template<typename U,U u> struct wrap_t;

  template<typename U> static yes check( wrap_t<U,U(0)>* );

  struct X { X(int); };
  int main() {
    check<X>(0);
  }

I can see 2 reasons this might cause type deduction to fail:

• Conversion is not a compile time constant.
• Non-type parameter is a class type.

Neither case is mentioned in 13.10.3 [temp.deduct] paragraph 2, nor do I see a DR mentioning these.

Proposed resolution (October 2002):

Add after the fourth-to-last bullet of 13.10.3 [temp.deduct] paragraph 2:

• Attempting to give an invalid type to a nontype template parameter. [Example:

    template <class T, T> struct S {};
    template <class T> int f(S<T, T()>*);
    struct X {};
    int i0 = f<X>(0);
  

  ]