It turned out that DR 2352 needs some fixing: as of now, it means that in
void f(int*); // #1
void f(const int* const &); // #2
void g(int* p) { f(p); }
the call to f is ambiguous. This broke e.g. Boost. I raised this on the CWG
reflector and Jason suggested wording that would break this up; this patch is
an attempt to implement that suggestion.
Jason, you remarked that adding a ck_qual under the ck_ref_bind might be
too much trouble, but it seems it's actually fairly simple. The comments
hopefully explain my thinking. Is this what you had in mind?
ref-bind6.C is something reduced from libstdc++ I came across when testing
this patch.
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2019-09-26 Marek Polacek
PR c++/91889 - follow-up fix for DR 2352.
* call.c (involves_qualification_conversion_p): New function.
(direct_reference_binding): Build a ck_qual if the conversion
would involve a qualification conversion.
(convert_like_real): Strip the conversion created by the ck_qual
in direct_reference_binding.
* g++.dg/cpp0x/ref-bind3.C: Add dg-error.
* g++.dg/cpp0x/ref-bind4.C: New test.
* g++.dg/cpp0x/ref-bind5.C: New test.
* g++.dg/cpp0x/ref-bind6.C: New test.
* g++.old-deja/g++.pt/spec35.C: Revert earlier change.
diff --git gcc/cp/call.c gcc/cp/call.c
index 45b984ecb11..09976369aed 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -1555,6 +1555,27 @@ reference_compatible_p (tree t1, tree t2)
return true;
}
+/* Return true if converting FROM to TO would involve a qualification
+ conversion. */
+
+static bool
+involves_qualification_conversion_p (tree to, tree from)
+{
+ /* If we're not convering a pointer to another one, we won't get
+ a qualification conversion. */
+ if (!((TYPE_PTR_P (to) && TYPE_PTR_P (from))
+ || (TYPE_PTRDATAMEM_P (to) && TYPE_PTRDATAMEM_P (from
+return false;
+
+ conversion *conv = standard_conversion (to, from, NULL_TREE,
+ /*c_cast_p=*/false, 0, tf_none);
+ for (conversion *t = conv; t; t = next_conversion (t))
+if (t->kind == ck_qual)
+ return true;
+
+ return false;
+}
+
/* A reference of the indicated TYPE is being bound directly to the
expression represented by the implicit conversion sequence CONV.
Return a conversion sequence for this binding. */
@@ -1598,6 +1619,19 @@ direct_reference_binding (tree type, conversion *conv)
That way, convert_like knows not to generate a temporary. */
conv->need_temporary_p = false;
}
+ else if (involves_qualification_conversion_p (t, conv->type))
+/* Represent the qualification conversion. After DR 2352
+ #1 and #2 were indistinguishable conversion sequences:
+
+void f(int*); // #1
+void f(const int* const &); // #2
+void g(int* p) { f(p); }
+
+ 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);
return build_conv (ck_ref_bind, type, conv);
}
@@ -7342,6 +7376,16 @@ convert_like_real (conversion *convs, tree expr, tree
fn, int argnum,
{
tree ref_type = totype;
+ /* direct_reference_binding might have inserted a ck_qual under
+ this ck_ref_bind for the benefit of conversion sequence ranking.
+ Ignore the conversion; we'll create our own below. */
+ if (next_conversion (convs)->kind == ck_qual)
+ {
+ gcc_assert (same_type_p (TREE_TYPE (expr),
+next_conversion (convs)->type));
+ STRIP_NOPS (expr);
+ }
+
if (convs->bad_p && !next_conversion (convs)->bad_p)
{
tree extype = TREE_TYPE (expr);
diff --git gcc/testsuite/g++.dg/cpp0x/ref-bind3.C
gcc/testsuite/g++.dg/cpp0x/ref-bind3.C
index 16e1bfe6ccc..b2c85ec684a 100644
--- gcc/testsuite/g++.dg/cpp0x/ref-bind3.C
+++ gcc/testsuite/g++.dg/cpp0x/ref-bind3.C
@@ -1,22 +1,18 @@
// PR c++/91844 - Implement CWG 2352, Similar types and reference binding.
// { dg-do compile { target c++11 } }
-template int f (const T *const &); // (1)
-template int f (T *const &); // (2)
-template int f (T *); // (3)
-
-/* Before CWG 2352, (2) was a better match than (1), but (2) and (3) were
- equally good, so there was an ambiguity. (2) was better than (1) because
- (1) required a qualification conversion whereas (2) didn't. But with this
- CWG, (1) no longer requires a qualification conversion, because the types
- "const int* const" and "int *" are now considered reference-related and we
- bind directly, and (1) is more specialized than (2). And (1) is also a
- better match than (3). */
+template int f (const T *const &); // 1
+template int f (T
