[clang] [clang][serialization] Fix crash on imported pack indexing type (PR #205965)

[via cfe-commits]

https://github.com/IamYJLee updated https://github.com/llvm/llvm-project/pull/205965

>From e4e0af6e75a56918530aaf31c6024de7cdb1c09a Mon Sep 17 00:00:00 2001
From: LeeYoungJoon <dog3hk.dev at gmail.com>
Date: Fri, 26 Jun 2026 12:39:01 +0900
Subject: [PATCH 1/2] [clang][ast] Add an index property for PackIndexingType
 serialization

---
 clang/include/clang/AST/TypeProperties.td | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/clang/include/clang/AST/TypeProperties.td b/clang/include/clang/AST/TypeProperties.td
index f16c10da430f9..e2168d0a00ff4 100644
--- a/clang/include/clang/AST/TypeProperties.td
+++ b/clang/include/clang/AST/TypeProperties.td
@@ -489,9 +489,12 @@ let Class = PackIndexingType in {
   def : Property<"expansions", Array<QualType>> {
     let Read = [{ node->getExpansions() }];
   }
+  def : Property<"index", UnsignedOrNone> {
+    let Read = [{ node->getSelectedIndex() }];
+  }
 
   def : Creator<[{
-    return ctx.getPackIndexingType(pattern, indexExpression, isFullySubstituted, expansions);
+    return ctx.getPackIndexingType(pattern, indexExpression, isFullySubstituted, expansions, index);
   }]>;
 }
 

>From ff931c9c8d399a1c7e4c45e9b12d45b52ef1e483 Mon Sep 17 00:00:00 2001
From: LeeYoungJoon <dog3hk.dev at gmail.com>
Date: Mon, 29 Jun 2026 11:28:03 +0900
Subject: [PATCH 2/2] Add testcase and release note

---
 clang/docs/ReleaseNotes.rst          |  1 +
 clang/test/Modules/pack-indexing.cpp | 31 ++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)
 create mode 100644 clang/test/Modules/pack-indexing.cpp

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a2439ccb0452a..53b57666e0f87 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -816,6 +816,7 @@ Bug Fixes to C++ Support
 - Fixed an issue where Clang incorrectly accepted invalid unqualified uses of local nested class names outside their declaring scope. (#GH184622)
 - Fixed a crash when parsing invalid friend declaration with storage-class specifier. (#GH186569)
 - Fixed a missing vtable for ``dynamic_cast<FinalClass *>(this)`` in a function template. (#GH198511)
+- Fixed a crash when using a pack indexing type (e.g. ``Ts...[0]``) imported from another module. (#GH204479)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/test/Modules/pack-indexing.cpp b/clang/test/Modules/pack-indexing.cpp
new file mode 100644
index 0000000000000..1908f535577a3
--- /dev/null
+++ b/clang/test/Modules/pack-indexing.cpp
@@ -0,0 +1,31 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++26 -triple %itanium_abi_triple -emit-module-interface %t/a.cppm -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++26 -triple %itanium_abi_triple -fmodule-file=a=%t/a.pcm -emit-llvm -o - %t/b.cpp | FileCheck %s
+
+// A fully-substituted pack indexing type (`Ts...[0]`) exported through a module
+// used to lose its selected index on deserialization, so its canonical type was
+// rebuilt as a dependent PackIndexingType. CodeGen then hit
+// llvm_unreachable("Non-canonical or dependent types aren't possible.").
+
+//--- a.cppm
+export module a;
+
+template<typename... Ts>
+using element = Ts...[0];
+
+export element<int> a = 0;
+
+//--- b.cpp
+import a;
+
+int b() {
+  return a;
+}
+
+// The pack indexing type `element<int>` must be lowered to `i32` without
+// crashing in CodeGen.
+// CHECK: @_ZW1a1a = external global i32
+// CHECK: define {{.*}}i32 @_Z1bv()
+// CHECK: load i32, ptr @_ZW1a1a