Ping.
On Fri, May 10, 2019 at 07:10:16PM -0400, 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,
> "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;
> +}
Marek
