c++: requires-expr in pack expansion using pack [PR103105]

Here after dependent substitution of {Ts...} into the alias 'wrap',
since we never partially instantiate a requires-expr, we end up with a
requires-expr whose REQUIRES_EXPR_EXTRA_ARGS contains an
ARGUMENT_PACK_SELECT (which just resolves to the parameter pack Ts).
Then when hashing the resulting dependent specialization of A, we crash
from iterative_hash_template_arg since it deliberately doesn't handle
ARGUMENT_PACK_SELECT.

Like in r12-7102-gdb5f1c17031ad8, it seems the right fix here is to
resolve ARGUMENT_PACK_SELECT arguments before storing them into an
extra args tree (such as REQUIRES_EXPR).

	PR c++/103105

gcc/cp/ChangeLog:

	* pt.c (build_extra_args): Call preserve_args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-requires29.C: New test.
	* g++.dg/cpp2a/concepts-requires29a.C: New test.

(cherry picked from commit e2c7070ac7740508a7c49bfee9f895e216a272d6)

3 files changed, 42 insertions, 1 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index da289c2..40a3f19 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13007,7 +13007,7 @@ build_extra_args (tree pattern, tree args, tsubst_flags_t complain)
 {
   /* Make a copy of the extra arguments so that they won't get changed
      out from under us.  */
-  tree extra = copy_template_args (args);
+  tree extra = preserve_args (copy_template_args (args), /*cow_p=*/false);
   if (local_specializations)
     if (tree locals = extract_local_specs (pattern, complain))
       extra = tree_cons (NULL_TREE, extra, locals);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires29.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires29.C
new file mode 100644
index 0000000..2cf6943
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires29.C
@@ -0,0 +1,18 @@
+// PR c++/103105
+// { dg-do compile { target c++20 } }
+
+template<bool> struct A;
+
+template<class... Ts>
+using wrap = A<1 != (0 + ... + requires { Ts(); })>;
+
+template<class... Ts> using type = wrap<Ts...>;
+
+using ty0 = type<>;
+using ty0 = A<true>;
+
+using ty1 = type<int>;
+using ty1 = A<false>;
+
+using ty2 = type<int, int>;
+using ty2 = A<true>;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires29a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires29a.C
new file mode 100644
index 0000000..84c2269
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires29a.C
@@ -0,0 +1,23 @@
+// PR c++/103105
+// { dg-do compile { target c++20 } }
+
+template<class...> struct list;
+
+template<bool> struct A;
+
+template<class T, class... Ts>
+using wrap = A<1 != (0 + ... + requires { T() = Ts(); })>;
+
+template<class... Ts> using type = list<wrap<Ts, Ts...>...>;
+
+using ty0 = type<>;
+using ty0 = list<>;
+
+using ty1 = type<int>;
+using ty1 = list<A<true>>;
+
+using ty2 = type<int, int>;
+using ty2 = list<A<true>, A<true>>;
+
+using ty3 = type<int, int, int>;
+using ty3 = list<A<true>, A<true>, A<true>>;