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

Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in
param4.C below due to a spurious constness mismatch between the type of
the template argument &f and the substituted type of the parm F (the
latter 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: we're not dropping top-level cv-quals on the substituted
type of an NTTP as per [temp.param]/6 (we only do so at parse time in
process_template_parm).  So this patch makes convert_template_argument
drop top-level cv-quals accordingly.

	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.
	* g++.dg/cpp1z/nontype-auto19.C: New test.
	* g++.dg/cpp2a/concepts-decltype.C: Don't expect that the
	deduced type of a decltype(auto) NTTP has top-level cv-quals.

5 files changed, 30 insertions, 1 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 744461e..2ae886d 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8499,6 +8499,10 @@ convert_template_argument (tree parm,
       if (invalid_nontype_parm_type_p (t, complain))
 	return error_mark_node;
 
+      /* Drop top-level cv-qualifiers on the substituted/deduced type of
+	 this non-type template parameter, as per [temp.param]/6.  */
+      t = cv_unqualified (t);
+
       if (t != TREE_TYPE (parm))
 	t = canonicalize_type_argument (t, complain);
 
diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C b/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
new file mode 100644
index 0000000..d6b904f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
@@ -0,0 +1,8 @@
+// Verify top-level cv-qualifiers are dropped from the deduced
+// type of a non-type template parameter, as per [temp.param]/6.
+// { dg-do compile { target c++17 } }
+
+constexpr int x = 42;
+template<decltype(auto) V> decltype(V)& f();
+using type = decltype(f<x>());
+using type = int&;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
index 13733c6..b375f74 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
@@ -61,7 +61,7 @@ constexpr int Z = 10;
 
 static_assert(deduced_as<0, int>);
 static_assert(deduced_as<0, int&>); // { dg-error "invalid variable template" }
-static_assert(deduced_as<Z, const int>);
+static_assert(deduced_as<Z, int>);
 static_assert(deduced_as<(Z), const int>); // { dg-error "invalid variable template" }
 static_assert(deduced_as<(Z), const int&>);
 
diff --git a/gcc/testsuite/g++.dg/template/param4.C b/gcc/testsuite/g++.dg/template/param4.C
new file mode 100644
index 0000000..8061ff7
--- /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 0000000..89a5c04
--- /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&;