From: Patrick Palka <ppalka@redhat.com>
Date: Thu, 5 Feb 2026 14:13:42 +0000 (-0500)
Subject: c++: cv-qualified pack index propagation [PR122169]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6385a3582c0b6b30479bc68f7d2b74e327ec44d7;p=thirdparty%2Fgcc.git

c++: cv-qualified pack index propagation [PR122169]

Since type pack indexes can be cv-qualified, we need to propagate their
qualifiers when substituting them.

	PR c++/122169

gcc/cp/ChangeLog:

	* pt.cc (tsubst_pack_index): Propagate cv-qualifiers of
	PACK_INDEX_TYPE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/pack-indexing19.C: New test.

Reviewed-by: Marek Polacek <polacek@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
---

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1ee8e2fff7d..08c6b42ef05 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -14368,10 +14368,15 @@ tsubst_pack_index (tree t, tree args, tsubst_flags_t complain, tree in_decl)
   tree index = tsubst_expr (PACK_INDEX_INDEX (t), args, complain, in_decl);
   const bool parenthesized_p = (TREE_CODE (t) == PACK_INDEX_EXPR
 				&& PACK_INDEX_PARENTHESIZED_P (t));
+  tree r;
   if (!value_dependent_expression_p (index) && TREE_CODE (pack) == TREE_VEC)
-    return pack_index_element (index, pack, parenthesized_p, complain);
+    r = pack_index_element (index, pack, parenthesized_p, complain);
   else
-    return make_pack_index (pack, index);
+    r = make_pack_index (pack, index);
+  if (TREE_CODE (t) == PACK_INDEX_TYPE)
+    r = cp_build_qualified_type (r, cp_type_quals (t) | cp_type_quals (r),
+				 complain | tf_ignore_bad_quals);
+  return r;
 }
 
 /* Make an argument pack out of the TREE_VEC VEC.  */
diff --git a/gcc/testsuite/g++.dg/cpp26/pack-indexing19.C b/gcc/testsuite/g++.dg/cpp26/pack-indexing19.C
new file mode 100644
index 00000000000..cd937057908
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/pack-indexing19.C
@@ -0,0 +1,32 @@
+// PR c++/122169
+// { dg-do compile { target c++26 } }
+
+template<class T, class U>
+concept same_as = __is_same(T, U);
+
+template<class... Ts>
+void f() {
+  using T = Ts...[0];
+  static_assert(same_as<const T, const int>);
+
+  []<int I>() {
+    using U = Ts...[I];
+    static_assert(same_as<const U, const int>);
+  }.template operator()<0>();
+}
+
+template void f<int>();
+
+
+template<class... Ts>
+void g() {
+  using T = Ts...[0];
+  static_assert(same_as<const T, const volatile int>);
+
+  []<int I>() {
+    using U = Ts...[I];
+    static_assert(same_as<const U, const volatile int>);
+  }.template operator()<0>();
+}
+
+template void g<volatile int>();