c++: NTTP with array/function type after substitution [PR61355]

We're performing the [temp.param]/10 adjustment at parse time but not
also at substitution time.

	PR c++/61355

gcc/cp/ChangeLog:

	* pt.c (convert_template_argument): Perform array/function to
	pointer conversion on the substituted type of an NTTP.

gcc/testsuite/ChangeLog:

	* g++.old-deja/g++.pt/nontype5.C: Adjust.
	* g++.dg/template/param6.C: New test.

gcc/cp/pt.c

@@ -8530,6 +8530,10 @@ convert_template_argument (tree parm,
       else
 	t = tsubst (t, args, complain, in_decl);
 
+      /* Perform array-to-pointer and function-to-pointer conversion
+	 as per [temp.param]/10.  */
+      t = type_decays_to (t);
+
       if (invalid_nontype_parm_type_p (t, complain))
 	return error_mark_node;
 

gcc/testsuite/g++.dg/template/param6.C

@@ -0,0 +1,32 @@
+// PR c++/61355
+// Verify we perform array-to-pointer and function-to-pointer conversion
+// on the substituted/deduced type of an NTTP.
+
+int f();
+int p[5];
+
+namespace cpp98 {
+  template<class T, T> struct X;
+  typedef X<int(), f> ty1;
+  typedef X<int[5], p> ty2;
+}
+
+namespace cpp11 {
+#if __cpp_variadic_templates
+  template<class T, T...> struct X;
+  using ty1 = X<int(), f>;
+  using ty2 = X<int[5], p>;
+#endif
+}
+
+namespace cpp17 {
+#if __cpp_nontype_template_parameter_auto
+  template<decltype(auto)> struct X;
+  using ty1 = X<f>;
+  using ty2 = X<p>;
+
+  template<decltype(auto)...> struct Y;
+  using ty3 = Y<f>;
+  using ty4 = Y<p>;
+#endif
+}

gcc/testsuite/g++.old-deja/g++.pt/nontype5.C

@@ -19,5 +19,5 @@ static int g() { return f(); }
 int f() { return 0; }
 
 int main() {
-return B<int,&f>::g();  // { dg-error "" } could not convert arg
+return B<int,&f>::g();
 }