https://github.com/ahatanak created 
https://github.com/llvm/llvm-project/pull/170360

operator[] can potentially cause reallocation and invalidate live iterators if 
it's called with a key that isn't present in the DenseMap. Call lookup() 
instead to prevent the function from inserting new entries into the DenseMap 
for ObjC classes that don't have any subclasses.

rdar://165448332

>From 65fa4c46201be7077c8402eba791cf5c984cd98c Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <[email protected]>
Date: Tue, 2 Dec 2025 12:08:16 -0800
Subject: [PATCH] Fix a use-after-free crash in ResetObjCLayout

operator[] can potentially cause reallocation and invalidate live
iterators if it's called with a key that isn't present in the DenseMap.
Call lookup() instead to prevent the function from inserting new entries
into the DenseMap for ObjC classes that don't have any subclasses.

rdar://165448332
---
 clang/lib/AST/ASTContext.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index b359fc8350375..404ce3ffd77c7 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -12040,7 +12040,7 @@ bool ASTContext::mergeExtParameterInfo(
 void ASTContext::ResetObjCLayout(const ObjCInterfaceDecl *D) {
   if (auto It = ObjCLayouts.find(D); It != ObjCLayouts.end()) {
     It->second = nullptr;
-    for (auto *SubClass : ObjCSubClasses[D])
+    for (auto *SubClass : ObjCSubClasses.lookup(D))
       ResetObjCLayout(SubClass);
   }
 }

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