This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1671. Unclear rules for deduction with cv-qualification

1. Simple pointer:

Template argument: const T*
Actual argument: char*
We deduce T to be char, and by compatibility of cv-qualifiers, this deduction works.

Now suppose that (somehow) we deduced T to be const char. We fold the resulting const const char* into const char*, and again the deduction works. Does the standard disallow this deduction? The reason for this question is in the next example.

2. Pointer to member:

Template argument: const T A<T>::*, where A is a class template
Actual argument: char A<const char>::*
Compilers reject this case, because if we deduce the first T to be char, we cannot match A<T>::*. But suppose we look first at the second T, deducing it to be const char. Then we get const char A<const char>::*, which is OK. Alternatively, as in the hypothetical case in example 1, suppose we deduce the first T to be const char, again we get a match.

Arbitrarily adding extra cv-qualifiers in looking for a match, or trying different matching orders to find one that works, seems wrong. But are these approaches disallowed?

For completeness, here is a code example:

  template <typename Q>
  struct A {
    int i;
  };

  template <typename T>
  void foo(const T A<T>::*) { }

  int main() {
    A<const int> a;
    int A<const int>::*p = &A<const int>::i;
    foo(p);  // rejected by all compilers, but why?
  }

Rationale (September, 2013):

There are no rules that would result in deducing const char in the specified example.