https://github.com/dmpolukhin updated 
https://github.com/llvm/llvm-project/pull/123648

>From e5cd06ddbc4193f9d5910eba93f0eb309d67063c Mon Sep 17 00:00:00 2001
From: Dmitry Polukhin <dmitry.poluk...@gmail.com>
Date: Mon, 20 Jan 2025 09:03:25 -0800
Subject: [PATCH 1/3] [C++20][Modules] Fix crash/compiler error due broken AST
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Summary:
This PR fixes bugreport https://github.com/llvm/llvm-project/issues/122493
The root problem is the same as before lambda function and DeclRefExpr 
references
a variable that does not belong to the same module as the enclosing function 
body.
Therefore iteration over the function body doesn’t visit the VarDecl. Before 
this
change RelatedDeclsMap was created only for canonical decl but in reality it 
has to
be done for the definition of the function that does not always match the 
canonical decl.

Test Plan: check-clang
---
 clang/lib/Serialization/ASTWriterDecl.cpp     |  5 +-
 ...ash-instantiated-in-scope-cxx-modules5.cpp | 92 +++++++++++++++++++
 2 files changed, 95 insertions(+), 2 deletions(-)
 create mode 100644 
clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp 
b/clang/lib/Serialization/ASTWriterDecl.cpp
index f8ed155ca389d7..371b4b29e85991 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1571,9 +1571,10 @@ void ASTDeclWriter::VisitCXXRecordDecl(CXXRecordDecl *D) 
{
     } else {
       Record.push_back(0);
     }
-    // For lambdas inside canonical FunctionDecl remember the mapping.
+    // For lambdas inside template functions, remember the mapping to
+    // deserialize them together.
     if (auto FD = llvm::dyn_cast_or_null<FunctionDecl>(D->getDeclContext());
-        FD && FD->isCanonicalDecl()) {
+        FD && FD->isDependentContext() && FD->isThisDeclarationADefinition()) {
       Writer.RelatedDeclsMap[Writer.GetDeclRef(FD)].push_back(
           Writer.GetDeclRef(D));
     }
diff --git a/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp 
b/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp
new file mode 100644
index 00000000000000..ca80a3fd21dd00
--- /dev/null
+++ b/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp
@@ -0,0 +1,92 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -verify -std=c++20 -xc++ -emit-module module.cppmap 
-fmodule-name=mock_resolver -o mock_resolver.pcm
+// RUN: %clang_cc1 -verify -std=c++20 -xc++ -emit-module module.cppmap 
-fmodule-name=sql_internal -o sql_internal.pcm
+// RUN: %clang_cc1 -verify -std=c++20 -xc++ -fmodule-file=mock_resolver.pcm 
-fmodule-file=sql_internal.pcm main.cc -o main.o
+
+//--- module.cppmap
+module "mock_resolver" {
+  export *
+  module "mock_resolver.h" {
+    export *
+    header "mock_resolver.h"
+  }
+}
+
+module "sql_internal" {
+  export *
+  module "sql_transform_builder.h" {
+    export *
+    header "sql_transform_builder.h"
+  }
+}
+
+//--- set_bits2.h
+// expected-no-diagnostics
+#pragma once
+
+template <typename T>
+void fwd(const T& x) {}
+
+namespace vox::bitset {
+
+template <typename TFunc>
+void ForEachSetBit2(const TFunc&) {
+  fwd([](int) {
+    const int bit_index_base = 0;
+    (void)[&](int) {
+      int v = bit_index_base;
+    };
+  });
+}
+
+}  // namespace vox::bitset
+
+//--- sql_transform_builder.h
+// expected-no-diagnostics
+#pragma once
+
+#include "set_bits2.h"
+
+class QualifyingSet3 {
+ public:
+  void GetIndexes() const {
+    vox::bitset::ForEachSetBit2([]() {});
+  }
+};
+
+template <typename T>
+void DoTransform() {
+  vox::bitset::ForEachSetBit2([]() {});
+}
+
+//--- mock_resolver.h
+// expected-no-diagnostics
+#pragma once 
+#include "set_bits2.h"
+
+class QualifyingSet2 {
+ public:
+  void GetIndexes() const {
+    vox::bitset::ForEachSetBit2([]() {});
+  }
+};
+
+//--- main.cc
+// expected-no-diagnostics
+#include "sql_transform_builder.h"
+
+template <typename Callable>
+void get(const Callable& fn) {
+  fwd<Callable>(fn);
+}
+
+namespace {
+
+void test() {
+  get([]() {});
+  DoTransform<int>();
+}
+
+} // namespace

>From 01254792e2b75e00cc44259b27fcb4e38e719629 Mon Sep 17 00:00:00 2001
From: Dmitry Polukhin <dmitry.poluk...@gmail.com>
Date: Tue, 21 Jan 2025 02:32:33 -0800
Subject: [PATCH 2/3] Add -Werror=uninitialized to add compilation error in
 builds without assserts All improve comments

---
 clang/include/clang/Serialization/ASTReader.h         | 11 ++++++-----
 clang/lib/Serialization/ASTWriterDecl.cpp             |  6 +++---
 .../crash-instantiated-in-scope-cxx-modules5.cpp      |  6 +++---
 3 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/clang/include/clang/Serialization/ASTReader.h 
b/clang/include/clang/Serialization/ASTReader.h
index 9f978762a6fb6b..d27a49863fc557 100644
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -539,11 +539,12 @@ class ASTReader
 
   /// Mapping from main decl ID to the related decls IDs.
   ///
-  /// These related decls have to be loaded right after the main decl.
-  /// It is required to have canonical declaration for related decls from the
-  /// same module as the enclosing main decl. Without this, due to lazy
-  /// deserialization, canonical declarations for the main decl and related can
-  /// be selected from different modules.
+  /// The key is the main decl ID, and the value is a vector of related decls
+  /// that must be loaded immediately after the main decl. This is necessary
+  /// to ensure that the definition for related decls comes from the same 
module
+  /// as the enclosing main decl. Without this, due to lazy deserialization,
+  /// the definition for the main decl and related decls may come from 
different
+  /// modules.
   llvm::DenseMap<GlobalDeclID, SmallVector<GlobalDeclID, 4>> RelatedDeclsMap;
 
   struct PendingUpdateRecord {
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp 
b/clang/lib/Serialization/ASTWriterDecl.cpp
index 371b4b29e85991..9835f58543e179 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -799,9 +799,9 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
   }
 
   if (D->getFriendObjectKind()) {
-    // For a function defined inline within a class template, we have to force
-    // the canonical definition to be the one inside the canonical definition 
of
-    // the template. Remember this relation to deserialize them together.
+    // For a friend function defined inline within a class template, we have to
+    // force the definition to be the one inside the definition of the template
+    // class. Remember this relation to deserialize them together.
     if (auto *RD = dyn_cast<CXXRecordDecl>(D->getLexicalParent()))
       if (RD->isDependentContext() && RD->isThisDeclarationADefinition()) {
         Writer.RelatedDeclsMap[Writer.GetDeclRef(RD)].push_back(
diff --git a/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp 
b/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp
index ca80a3fd21dd00..352e0125fe4342 100644
--- a/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp
+++ b/clang/test/Headers/crash-instantiated-in-scope-cxx-modules5.cpp
@@ -1,9 +1,9 @@
 // RUN: rm -fR %t
 // RUN: split-file %s %t
 // RUN: cd %t
-// RUN: %clang_cc1 -verify -std=c++20 -xc++ -emit-module module.cppmap 
-fmodule-name=mock_resolver -o mock_resolver.pcm
-// RUN: %clang_cc1 -verify -std=c++20 -xc++ -emit-module module.cppmap 
-fmodule-name=sql_internal -o sql_internal.pcm
-// RUN: %clang_cc1 -verify -std=c++20 -xc++ -fmodule-file=mock_resolver.pcm 
-fmodule-file=sql_internal.pcm main.cc -o main.o
+// RUN: %clang_cc1 -verify -std=c++20 -Werror=uninitialized -xc++ -emit-module 
module.cppmap -fmodule-name=mock_resolver -o mock_resolver.pcm
+// RUN: %clang_cc1 -verify -std=c++20 -Werror=uninitialized -xc++ -emit-module 
module.cppmap -fmodule-name=sql_internal -o sql_internal.pcm
+// RUN: %clang_cc1 -verify -std=c++20 -Werror=uninitialized -xc++ 
-fmodule-file=mock_resolver.pcm -fmodule-file=sql_internal.pcm main.cc -o main.o
 
 //--- module.cppmap
 module "mock_resolver" {

>From 127e2fb66f25856c0d4653b75bdf13cf440f6955 Mon Sep 17 00:00:00 2001
From: Dmitry Polukhin <dmitry.poluk...@gmail.com>
Date: Wed, 22 Jan 2025 02:10:34 -0800
Subject: [PATCH 3/3] Extract check into template function.

---
 clang/lib/Serialization/ASTWriterDecl.cpp | 29 +++++++++++++++++------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp 
b/clang/lib/Serialization/ASTWriterDecl.cpp
index 9835f58543e179..e5bdd0ae43a6ed 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -27,6 +27,24 @@
 using namespace clang;
 using namespace serialization;
 
+//===----------------------------------------------------------------------===//
+// Utility functions
+//===----------------------------------------------------------------------===//
+
+namespace {
+
+// Helper function that returns dependent decl context for the declaration
+// passed in the argument.
+template <typename DCT> DCT *getDependentDeclContext(Decl *D) {
+  if (auto *DC = llvm::dyn_cast_or_null<DCT>(D->getDeclContext());
+      DC && DC->isDependentContext() && DC->isThisDeclarationADefinition()) {
+    return DC;
+  }
+  return nullptr;
+}
+
+} // namespace
+
 
//===----------------------------------------------------------------------===//
 // Declaration serialization
 
//===----------------------------------------------------------------------===//
@@ -802,11 +820,9 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
     // For a friend function defined inline within a class template, we have to
     // force the definition to be the one inside the definition of the template
     // class. Remember this relation to deserialize them together.
-    if (auto *RD = dyn_cast<CXXRecordDecl>(D->getLexicalParent()))
-      if (RD->isDependentContext() && RD->isThisDeclarationADefinition()) {
-        Writer.RelatedDeclsMap[Writer.GetDeclRef(RD)].push_back(
-            Writer.GetDeclRef(D));
-      }
+    if (auto *RD = getDependentDeclContext<CXXRecordDecl>(D))
+      Writer.RelatedDeclsMap[Writer.GetDeclRef(RD)].push_back(
+          Writer.GetDeclRef(D));
   }
 
   Record.push_back(D->param_size());
@@ -1573,8 +1589,7 @@ void ASTDeclWriter::VisitCXXRecordDecl(CXXRecordDecl *D) {
     }
     // For lambdas inside template functions, remember the mapping to
     // deserialize them together.
-    if (auto FD = llvm::dyn_cast_or_null<FunctionDecl>(D->getDeclContext());
-        FD && FD->isDependentContext() && FD->isThisDeclarationADefinition()) {
+    if (auto *FD = getDependentDeclContext<CXXRecordDecl>(D)) {
       Writer.RelatedDeclsMap[Writer.GetDeclRef(FD)].push_back(
           Writer.GetDeclRef(D));
     }

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