[clang] ad5add5 - [clang][serialization] Fix crash on imported pack indexing type (#205965)

[via cfe-commits]

Author: LeeYoungJoon
Date: 2026-07-02T16:21:55+09:00
New Revision: ad5add5fb62573fc1f139a40658d616e16a82625

URL: https://github.com/llvm/llvm-project/commit/ad5add5fb62573fc1f139a40658d616e16a82625
DIFF: https://github.com/llvm/llvm-project/commit/ad5add5fb62573fc1f139a40658d616e16a82625.diff

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

The selected index of a PackIndexingType was not serialized, so on
deserialization its canonical type was rebuilt as a dependent type,
crashing CodeGen.

Difference in `Ty` during `CodeGenTypes::ConvertType`
(https://github.com/llvm/llvm-project/blob/49cf5a7ba0ab76a1f700ce6407cea1713741bc01/clang/lib/CodeGen/CodeGenTypes.cpp#L415-L424)

### Before 
```cpp
PackIndexingType 0x142914680 'int' sugar
|-SubstTemplateTypeParmPackType 0x1429145a0 'Ts' dependent contains_unexpanded_pack imported typename depth 0 index 0 ... Ts
| |-TypeAliasTemplate 0x142914318 'element'
| `-TemplateArgument pack '<int>'
|   `-TemplateArgument type 'int'
|     `-BuiltinType 0x14401cf10 'int'
|-ConstantExpr 0x142914630 '__size_t':'unsigned long'
| |-value: Int 0
| `-ImplicitCastExpr 0x1429145f0 '__size_t':'unsigned long' <IntegralCast>
|   `-IntegerLiteral 0x1429145d0 'int' 0
`-SubstTemplateTypeParmType 0x142914650 'int' sugar imported typename depth 0 index 0 ... Ts pack_index 0 final
  |-TypeAliasTemplate 0x142914318 'element'
  `-BuiltinType 0x14401cf10 'int'
```

### After
```cpp
BuiltinType 0x14e02e710 'int'
```

I used AI assistance when writing the test code, but I personally
reviewed it. 🤖

Fixes https://github.com/llvm/llvm-project/issues/204479

---------

Co-authored-by: Younan Zhang <zyn7109 at gmail.com>

Added: 
    clang/test/Modules/pr204479.cppm

Modified: 
    clang/docs/ReleaseNotes.md
    clang/include/clang/AST/TypeProperties.td

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index af80d8b9d0495..21b2d10d05bc1 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -830,6 +830,7 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the
 - 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 an assertion failure during init-list checking of an array whose element type is an incomplete class. (#GH140685)
+- 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/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);
   }]>;
 }
 

diff  --git a/clang/test/Modules/pr204479.cppm b/clang/test/Modules/pr204479.cppm
new file mode 100644
index 0000000000000..8a636aed9cf17
--- /dev/null
+++ b/clang/test/Modules/pr204479.cppm
@@ -0,0 +1,24 @@
+// 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.cppm
+export module a;
+
+template<typename... Ts>
+using element = Ts...[0];
+
+export element<int> a = 0;
+
+//--- b.cpp
+import a;
+
+int b() {
+  return a;
+}
+
+// CHECK: @_ZW1a1a = external global i32
+// CHECK: define {{.*}}i32 @_Z1bv()
+// CHECK: load i32, ptr @_ZW1a1a