[gcc r11-9929] c++: ambiguous call not diagnosed after DR2352 [PR97296]

[gcc r11-9929] c++: ambiguous call not diagnosed after DR2352 [PR97296]

[Marek Polacek]

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)

Diff:
---
 gcc/cp/call.c                          | 15 +++++++++++++--
 gcc/testsuite/g++.dg/cpp0x/ref-bind4.C |  2 +-
 gcc/testsuite/g++.dg/cpp0x/ref-bind8.C | 10 ++++++++++
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 55bb9c45f8c..023e5e0f4b5 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -1683,8 +1683,19 @@ direct_reference_binding (tree type, conversion *conv)
        because the types "int *" and "const int *const" are
        reference-related and we were binding both directly and they
        had the same rank.  To break it up, we add a ck_qual under the
-       ck_ref_bind so that conversion sequence ranking chooses #1.  */
-    conv = build_conv (ck_qual, t, conv);
+       ck_ref_bind so that conversion sequence ranking chooses #1.
+
+       We strip_top_quals here which is also what standard_conversion
+       does.  Failure to do so would confuse comp_cv_qual_signature
+       into thinking that in
+
+	 void f(const int * const &); // #1
+	 void f(const int *); // #2
+	 int *x;
+	 f(x);
+
+       #2 is a better match than #1 even though they're ambiguous (97296).  */
+    conv = build_conv (ck_qual, strip_top_quals (t), conv);
 
   return build_conv (ck_ref_bind, type, conv);
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/ref-bind4.C b/gcc/testsuite/g++.dg/cpp0x/ref-bind4.C
index 85ac9fbfd79..d296d7c3b72 100644
--- a/gcc/testsuite/g++.dg/cpp0x/ref-bind4.C
+++ b/gcc/testsuite/g++.dg/cpp0x/ref-bind4.C
@@ -51,6 +51,6 @@ g (int *p, const int *pc, const int **q)
      similar  types  T1 and T2 (_conv.qual_), respectively, and the cv-
      qualification signature of type T1 is a proper subset of  the  cv-
      qualification signature of type T2  */
-  f8 (q);
+  f8 (q); // { dg-error "call of overloaded" }
   f9 (q);
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/ref-bind8.C b/gcc/testsuite/g++.dg/cpp0x/ref-bind8.C
new file mode 100644
index 00000000000..eee78fd5e74
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/ref-bind8.C
@@ -0,0 +1,10 @@
+// PR c++/97296
+// { dg-do compile }
+
+void f(const int * const &);
+void f(const int *);
+int *x;
+int main()
+{
+  f(x); // { dg-error "call of overloaded" }
+}