Improved comments and add nullptr check

Author: dmpolukhin

clang/include/clang/Serialization/ASTReader.h

@@ -544,7 +544,13 @@ class ASTReader
   /// 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.
+  /// modules. It is used for the following cases:
+  /// - Lambda inside a template function definition: The main declaration is
+  ///   the enclosing function, and the related declarations are the lambda
+  ///   declarations.
+  /// - Friend function defined inside a template CXXRecord declaration: The
+  ///   main declaration is the enclosing record, and the related declarations
+  ///   are the friend functions.
   llvm::DenseMap<GlobalDeclID, SmallVector<GlobalDeclID, 4>> RelatedDeclsMap;
 
   struct PendingUpdateRecord {

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -817,7 +817,7 @@ void ASTDeclWriter::VisitFunctionDecl(FunctionDecl *D) {
     // 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());
-        isDefinitionInDependentContext(RD)) {
+        RD && isDefinitionInDependentContext(RD)) {
       Writer.RelatedDeclsMap[Writer.GetDeclRef(RD)].push_back(
           Writer.GetDeclRef(D));
     }