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

[Jason Merrill]

On 5/10/19 7:10 PM, Marek Polacek wrote:

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,
+                 if (TREE_CODE (arg) == FUNCTION_TYPE)
+                   inform (input_location, "  template argument for "
+                           "non-type template parameter is treated as "
+                           "function type");

Maybe add "ambiguous" to the beginning?  OK either way.

Jason