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.cc (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.

gcc/cp/call.cc

@@ -1680,8 +1680,19 @@ 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);
 }

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);
 }

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" }
+}