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

2024-09-25

1374. Qualification conversion vs difference in reference binding

Section: 12.2.4.3  [over.ics.rank]     Status: CD3     Submitter: Michael Wong     Date: 2011-08-15

[Moved to DR at the April, 2013 meeting.]

The rule in 12.2.4.3 [over.ics.rank] paragraph 3 for ranking based on a difference in qualification conversion applies only if they "differ only in their qualification conversion".

It is unclear as to whether the property of being a reference binding is a factor in determining if there is a difference between conversion sequences. Notice that 12.2.4.2.5 [over.ics.ref] maps reference bindings to other forms of implicit conversion sequences, but does not state that the property of being a reference binding is preserved; however, 12.2.4.3 [over.ics.rank] has cases which depend on whether certain standard conversion sequences are reference bindings or not and on the specifics of the bindings.

In the following, picking T2 && would bind an rvalue to an rvalue reference. Picking T1 & would bind an rvalue to an lvalue reference, but the qualification conversion to T1 is "better". Which is better?

    typedef int *      *      *const *const T1;
    typedef int *const *const *const *const T2;
    void foo(T1 &);
    void foo(T2 &&) { }

    int main() {
       foo((int ****)0);
       return 0;
    }

Notes from the February, 2012 meeting:

The CWG agreed that bullets 3 and 4 should be reversed, to check the reference binding first and then for qualification conversion.

Proposed resolution (February, 2012):

Move 12.2.4.3 [over.ics.rank] paragraph 3, first bullet, third sub-bullet, after the current fifth sub-bullet, as follows:

Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies: