[C++20][Modules] Fix crash/compiler error due broken AST links

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 {

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(
@@ -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));
     }

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 -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" {
+  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