From 0bbf0edbfc782f8e4e416d5fbd1b52a515adb585 Mon Sep 17 00:00:00 2001
From: Patrick Palka <ppalka@redhat.com>
Date: Tue, 30 Mar 2021 22:54:37 -0400
Subject: [PATCH] c++: placeholder type constraint and argument pack [PR99815]

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 in order to properly separate the implicit 'auto' argument
from the rest.

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 a056ecefd1df..dc6f2f37f9bb 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 000000000000..eaea41a36eb6
--- /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;
+}
-- 
2.47.2