Re: [PATCH] c++: placeholder type constraint and argument packs [PR99815]

[Jason Merrill <jason@redhat.com>]

On 3/30/21 4:35 PM, Patrick Palka wrote:
> When checking dependence of a placeholder type constraint, if the first
> template argument of the constraint is an argument pack, we need to
> expand it so that we properly separate the implicit 'auto' argument from
> the rest.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> gcc/cp/ChangeLog:
> 
> 	PR c++/99815
> 	* pt.c (placeholder_type_constraint_dependent_p): Expand
> 	argument packs to separate the first non-pack argument
> 	from the rest.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/99815
> 	* g++.dg/cpp2a/concepts-placeholder5.C: New test.
> ---
>   gcc/cp/pt.c                                   |  5 +++
>   .../g++.dg/cpp2a/concepts-placeholder5.C      | 32 +++++++++++++++++++
>   2 files changed, 37 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder5.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index a056ecefd1d..dc6f2f37f9b 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -28189,6 +28189,11 @@ placeholder_type_constraint_dependent_p (tree t)
>     tree id = unpack_concept_check (t);
>     tree args = TREE_OPERAND (id, 1);
>     tree first = TREE_VEC_ELT (args, 0);
> +  if (ARGUMENT_PACK_P (first))
> +    {
> +      args = expand_template_argument_pack (args);
> +      first = TREE_VEC_ELT (args, 0);
> +    }
>     gcc_checking_assert (TREE_CODE (first) == WILDCARD_DECL
>   		       || is_auto (first));
>     for (int i = 1; i < TREE_VEC_LENGTH (args); ++i)
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder5.C b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder5.C
> new file mode 100644
> index 00000000000..eaea41a36eb
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder5.C
> @@ -0,0 +1,32 @@
> +// PR c++/99815
> +// { dg-do compile { target c++20 } }
> +
> +template <class T, class U>
> +struct is_same { static constexpr bool value = false; };
> +
> +template <class T>
> +struct is_same<T, T> { static constexpr bool value = true; };
> +
> +template <class... Ts>
> +concept C = is_same<Ts...>::value; // { dg-error "wrong number" }
> +
> +template <class... Ts> void f() {
> +  C<Ts...> auto x = 0; // { dg-error "constraints" }
> +}
> +
> +template void f<int>(); // { dg-bogus "" }
> +template void f<char>(); // { dg-message "required from here" }
> +template void f<>(); // { dg-message "required from here" }
> +template void f<int, int>(); // { dg-message "required from here" }
> +
> +template <class... Ts> void g() {
> +  C<Ts..., int> auto x = 0; // { dg-error "constraints" }
> +}
> +
> +template void g<>(); // { dg-bogus "" }
> +template void g<int>(); // { dg-message "required from here" }
> +
> +template <class> void h() {
> +  C<char> auto x = 0; // { dg-error "constraints" }
> +  C<int> auto y = 0;
> +}
>