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Subject: [Bug c++/97296] [10/11/12 Regression] g++ accepts-invalid after
 DR2352 fix
Date: Wed, 13 Apr 2022 12:42:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97296

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:85ae54e18b9a3cbe4feda921ecd77cf275177edf

commit r12-8138-g85ae54e18b9a3cbe4feda921ecd77cf275177edf
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 directl=
y.

    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 t=
wo
    ck_quals sees that "const int *" is a proper subset of "const int *cons=
t"
    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.cc (direct_reference_binding): strip_top_quals when crea=
ting
            a ck_qual.

    gcc/testsuite/ChangeLog:

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