C++ PATCH to improve diagnostic for non-type template parameters

[Marek Polacek]

When we have 

  template<auto>
  struct S { };

then in

   S<int()> s;

"int()" is resolved to a type-id, as per [temp.arg]/2, causing this program to
fail to compile.  This can be rather confusing so I think we want to improve the
diagnostic a bit. 

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-05-10  Marek Polacek  <pola...@redhat.com>

        * pt.c (convert_template_argument): Add a diagnostic for the
        [temp.arg]/2 ambiguity case.

        * g++.dg/cpp2a/nontype-class17.C: New test.

diff --git gcc/cp/pt.c gcc/cp/pt.c
index 08da94ae0c9..b38e65d7f7e 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -7961,10 +7961,22 @@ convert_template_argument (tree parm,
                     "parameter list for %qD",
                     i + 1, in_decl);
              if (is_type)
-               inform (input_location,
-                       "  expected a constant of type %qT, got %qT",
-                       TREE_TYPE (parm),
-                       (DECL_P (arg) ? DECL_NAME (arg) : orig_arg));
+               {
+                 /* The template argument is a type, but we're expecting
+                    an expression.  */
+                 inform (input_location,
+                         "  expected a constant of type %qT, got %qT",
+                         TREE_TYPE (parm),
+                         (DECL_P (arg) ? DECL_NAME (arg) : orig_arg));
+                 /* [temp.arg]/2: "In a template-argument, an ambiguity
+                    between a type-id and an expression is resolved to a
+                    type-id, regardless of the form of the corresponding
+                    template-parameter."  So give the user a clue.  */
+                 if (TREE_CODE (arg) == FUNCTION_TYPE)
+                   inform (input_location, "  template argument for "
+                           "non-type template parameter is treated as "
+                           "function type");
+               }
              else if (requires_tmpl_type)
                inform (input_location,
                        "  expected a class template, got %qE", orig_arg);
diff --git gcc/testsuite/g++.dg/cpp2a/nontype-class17.C 
gcc/testsuite/g++.dg/cpp2a/nontype-class17.C
new file mode 100644
index 00000000000..ca5f68e1611
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/nontype-class17.C
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++2a } }
+
+template<auto>
+struct S { };
+
+struct R { };
+
+void
+g (void)
+{
+  S<R()> s; // { dg-error "mismatch" }
+// { dg-message "treated as function" "note" { target *-*-* } .-1 }
+  S<R{}> s2;
+  S<int()> s3; // { dg-error "mismatch" }
+// { dg-message "treated as function" "note" { target *-*-* } .-1 }
+  S<int{}> s4;
+}