[Lldb-commits] [clang] [lldb] [lldb][TypeSystemClang] Initialize ClassTemplateSpecializationDecl's StrictPackMatch field (PR #126215)

[Michael Buch via lldb-commits]

https://github.com/Michael137 created https://github.com/llvm/llvm-project/pull/126215

This addresses the MSAN failure reported
in https://github.com/llvm/llvm-project/pull/125791#issuecomment-2639183154:
```
==5633==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 in clang::ASTNodeImporter::CallOverloadedCreateFun<clang::ClassTemplateSpecializationDecl>::operator()
    #1 in bool clang::ASTNodeImporter::GetImportedOrCreateSpecialDecl<...>
...
```

The ASTImporter reads `D->hasStrictPackMatch()` and forwards it to the constructor of the destination `ClassTemplateSpecializationDecl`. But if `D` is a decl that LLDB created from debug-info, it would've been created using `ClassTemplateSpecializationDecl::CreateDeserialized`, which doesn't initialize the `StrictPackMatch` field.

This patch just initializes the field to a fixed value of `false`, to preserve previous behaviour and avoid the use-of-uninitialized-value.

An alternative would be to always initialize it in the `ClassTemplateSpecializationDecl` constructor, but there were reservations about providing a default value for it because it might lead to hard-to-diagnose problems down the line.

>From b061996c0b00d9f219ebacaa2529d694d37b71a9 Mon Sep 17 00:00:00 2001
From: Michael Buch <michaelbuch12 at gmail.com>
Date: Fri, 7 Feb 2025 09:43:36 +0000
Subject: [PATCH] [lldb][TypeSystemClang] Initialize
 ClassTemplateSpecializationDecl's StrictPackMatch field

This addresses the MSAN failure reported
in https://github.com/llvm/llvm-project/pull/125791#issuecomment-2639183154:
```
==5633==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 in clang::ASTNodeImporter::CallOverloadedCreateFun<clang::ClassTemplateSpecializationDecl>::operator()
    #1 in bool clang::ASTNodeImporter::GetImportedOrCreateSpecialDecl<...>
...
```

The ASTImporter reads `D->hasStrictPackMatch()` and forwards it to the constructor of the destination `ClassTemplateSpecializationDecl`. But if `D` is a decl that LLDB created from debug-info, it would've been created using `ClassTemplateSpecializationDecl::CreateDeserialized`, which doesn't initialize the `StrictPackMatch` field.

This patch just initializes the field to a fixed value of `false`, to preserve previous behaviour and avoid the use-of-uninitialized-value.

An alternative would be to always initialize it in the `ClassTemplateSpecializationDecl` constructor, but there were reservations about providing a default value for it because it might lead to hard-to-diagnose problems down the line.
---
 clang/include/clang/AST/DeclTemplate.h                   | 2 ++
 lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index a30ae798a99bc69..b82f75dd63fa508 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -1960,6 +1960,8 @@ class ClassTemplateSpecializationDecl : public CXXRecordDecl,
 
   bool hasStrictPackMatch() const { return StrictPackMatch; }
 
+  void setStrictPackMatch(bool Val) { StrictPackMatch = Val; }
+
   /// Get the point of instantiation (if any), or null if none.
   SourceLocation getPointOfInstantiation() const {
     return PointOfInstantiation;
diff --git a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
index 1da8fbe0bcd6dda..ecb571b1161bbc6 100644
--- a/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
+++ b/lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp
@@ -1666,6 +1666,12 @@ TypeSystemClang::CreateClassTemplateSpecializationDecl(
   ast.getTypeDeclType(class_template_specialization_decl, nullptr);
   class_template_specialization_decl->setDeclName(
       class_template_decl->getDeclName());
+
+  // FIXME: set to fixed value for now so it's not uninitialized.
+  // One way to determine StrictPackMatch would be
+  // Sema::CheckTemplateTemplateArgument.
+  class_template_specialization_decl->setStrictPackMatch(false);
+
   SetOwningModule(class_template_specialization_decl, owning_module);
   decl_ctx->addDecl(class_template_specialization_decl);