[modules] Fix Decl's Used invariant.

The Decl::isUsed has a value for every decl. In non-module builds it is very
difficult (but possible) to break this invariant but when we walk up the redecl
chain we find the neccessary information.

When deserializing the decls from a module it is much more difficult to update
correctly this invariant. The patch centralizes the information whether a decl
is used in the canonical decl marking the entire entity as being used.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27401

Patch by Cristina Cristescu and me.

Thanks to Richard Smith who helped to debug and understand the issue!

Reviewed by Richard Smith.

llvm-svn: 267691

1 files changed, 7 insertions, 2 deletions

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 073ed674741..32c9c4787c4 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -5784,8 +5784,13 @@ void ASTWriter::AddedObjCCategoryToInterface(const ObjCCategoryDecl *CatD,
 
 void ASTWriter::DeclarationMarkedUsed(const Decl *D) {
   assert(!WritingAST && "Already writing the AST!");
-  if (!D->isFromASTFile())
-    return;
+
+  // If there is *any* declaration of the entity that's not from an AST file,
+  // we can skip writing the update record. We make sure that isUsed() triggers
+  // completion of the redeclaration chain of the entity.
+  for (auto Prev = D->getMostRecentDecl(); Prev; Prev = Prev->getPreviousDecl())
+    if (IsLocalDecl(Prev))
+      return;
 
   DeclUpdates[D].push_back(DeclUpdate(UPD_DECL_MARKED_USED));
 }