[PATCH] c++: top-level cv-quals on type of NTTP [PR100893]

[Patrick Palka via Gcc-patches]

Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in the
first testcase due to a spurious constness mismatch between the type of
the template argument &f and the substituted type of F (the substituted
type has a top-level const).  Note that this mismatch doesn't occur with
object pointers because in that case a call to
perform_qualification_conversions from convert_nontype_argument
implicitly adds a top-level const to the argument (via a cast) to match.

This however seems to be a manifestation of a more general conformance
issue -- that we're not dropping top-level cv-quals after substituting
into the type of an NTTP as per [temp.param]/6 (we only do so at parse
time in process_template_parm).  This patch makes convert_template_argument
drop top-level cv-quals accordingly.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

        PR c++/100893

gcc/cp/ChangeLog:

        * pt.c (convert_template_argument): Strip top-level cv-quals
        on the substituted type of a non-type template parameter.

gcc/testsuite/ChangeLog:

        * g++.dg/template/param4.C: New test.
        * g++.dg/template/param5.C: New test.
---
 gcc/cp/pt.c                            |  7 ++++++-
 gcc/testsuite/g++.dg/template/param4.C | 10 ++++++++++
 gcc/testsuite/g++.dg/template/param5.C |  7 +++++++
 3 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/template/param4.C
 create mode 100644 gcc/testsuite/g++.dg/template/param5.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 7211bdc5bbc..66cc88a331f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8494,7 +8494,12 @@ convert_template_argument (tree parm,
            return error_mark_node;
        }
       else
-       t = tsubst (t, args, complain, in_decl);
+       {
+         t = tsubst (t, args, complain, in_decl);
+         /* Ignore top-level qualifiers on the substituted type of this
+            non-type template parameter, as per [temp.param]/6.  */
+         t = cv_unqualified (t);
+       }
 
       if (invalid_nontype_parm_type_p (t, complain))
        return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/template/param4.C 
b/gcc/testsuite/g++.dg/template/param4.C
new file mode 100644
index 00000000000..d55fecec2d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/param4.C
@@ -0,0 +1,10 @@
+// PR c++/100893
+
+template<class T, typename T::type F> void g() { }
+
+struct A { typedef void (* const type)(); };
+void f();
+template void g<A, &f>();
+
+struct B { typedef void (B::* const type)(); void f(); };
+template void g<B, &B::f>();
diff --git a/gcc/testsuite/g++.dg/template/param5.C 
b/gcc/testsuite/g++.dg/template/param5.C
new file mode 100644
index 00000000000..b8c92f4c217
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/param5.C
@@ -0,0 +1,7 @@
+// Verify top-level cv-qualifiers are dropped when determining the substituted
+// type of a non-type-template parameter, as per [temp.param]/6.
+// { dg-do compile { target c++11 } }
+
+template<class T, T V> decltype(V)& f();
+using type = decltype(f<const volatile int, 0>());
+using type = int&;
-- 
2.32.0.rc2