[Bug c++/97296] [10/11 Regression] g++ accepts-invalid after DR2352 fix

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97296

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:47c74508116cb5959686b183374dfe97a5e4fb37

commit r11-9929-g47c74508116cb5959686b183374dfe97a5e4fb37
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Apr 12 17:30:30 2022 -0400

    c++: ambiguous call not diagnosed after DR2352 [PR97296]

    DR 2352 changed the definitions of reference-related (so that it uses
    "similar type" instead of "same type") and of reference-compatible (use
    a standard conversion sequence).  That means that reference-related is
    now more broad, which means that we will be binding more things directly.

    The original patch for DR 2352 caused some problems, which were fixed in
    r276251 by creating a "fake" ck_qual in direct_reference_binding, so
    that in

      void f(int *); // #1
      void f(const int * const &); // #2
      int *x;
      int main()
      {
        f(x); // call #1
      }

    we call #1.  The extra ck_qual in #2 causes compare_ics to select #1,
    which is a better match for "int *" because then we don't have to do
    a qualification conversion.

    Let's turn to the problem in this PR.  We have

      void f(const int * const &); // #1
      void f(const int *); // #2
      int *x;
      int main()
      {
        f(x);
      }

    We arrive in compare_ics to decide which one is better. The ICS for #1
    looks like

        ck_ref_bind      <-    ck_qual         <-   ck_identity
      const int *const &     const int *const         int *

    and the ICS for #2 is

        ck_qual     <-  ck_rvalue   <-  ck_identity
      const int *          int *           int *

    We strip the reference and then comp_cv_qual_signature when comparing two
    ck_quals sees that "const int *" is a proper subset of "const int *const"
    and we return -1.  But that's wrong; presumably the top-level "const"
    should be ignored and the call should be ambiguous.  This patch adjust
    the type of the "fake" ck_qual so that this problem doesn't arise.

            PR c++/97296

    gcc/cp/ChangeLog:

            * call.c (direct_reference_binding): strip_top_quals when creating
            a ck_qual.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/ref-bind4.C: Add dg-error.
            * g++.dg/cpp0x/ref-bind8.C: New test.

    (cherry picked from commit 85ae54e18b9a3cbe4feda921ecd77cf275177edf)