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

503. Cv-qualified function types in template argument deduction

Consider the following program:

    template <typename T> int ref (T&)                { return 0; }
    template <typename T> int ref (const T&)          { return 1; }
    template <typename T> int ref (const volatile T&) { return 2; }
    template <typename T> int ref (volatile T&)       { return 4; }

    template <typename T> int ptr (T*)                { return 0; }
    template <typename T> int ptr (const T*)          { return 8; }
    template <typename T> int ptr (const volatile T*) { return 16; }
    template <typename T> int ptr (volatile T*)       { return 32; }

    void foo() {}

    int main()
    {
        return ref(foo) + ptr(&foo);
    }

The Standard appears to specify that the value returned from main is 2. The reason for this result is that references and pointers are handled differently in template argument deduction.

For the reference case, 13.10.3.2 [temp.deduct.call] paragraph 3 says that “If P is a reference type, the type referred to by P is used for type deduction.” Because of issue 295, all four of the types for the ref function parameters are the same, with no cv-qualification; overload resolution does not find a best match among the parameters and thus the most-specialized function is selected.

For the pointer type, argument deduction does not get as far as forming a cv-qualified function type; instead, argument deduction fails in the cv-qualified cases because of the cv-qualification mismatch, and only the cv-unqualified version of ptr survives as a viable function.

I think the choice of ignoring cv-qualifiers in the reference case but not the pointer case is very troublesome. The reason is that when one considers function objects as function parameters, it introduces a semantic difference whether the function parameter is declared a reference or a pointer. In all other contexts, it does not matter: a function name decays to a pointer and the resulting semantics are the same.

(See also issue 1584.)