[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, 11 insertions, 7 deletions

diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index aec3b7cd3bd..338b059e53c 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -340,25 +340,29 @@ unsigned Decl::getMaxAlignment() const {
   return Align;
 }
 
-bool Decl::isUsed(bool CheckUsedAttr) const { 
-  if (Used)
+bool Decl::isUsed(bool CheckUsedAttr) const {
+  const Decl *CanonD = getCanonicalDecl();
+  if (CanonD->Used)
     return true;
-  
+
   // Check for used attribute.
-  if (CheckUsedAttr && hasAttr<UsedAttr>())
+  // Ask the most recent decl, since attributes accumulate in the redecl chain.
+  if (CheckUsedAttr && getMostRecentDecl()->hasAttr<UsedAttr>())
     return true;
 
-  return false; 
+  // The information may have not been deserialized yet. Force deserialization
+  // to complete the needed information.
+  return getMostRecentDecl()->getCanonicalDecl()->Used;
 }
 
 void Decl::markUsed(ASTContext &C) {
-  if (Used)
+  if (isUsed())
     return;
 
   if (C.getASTMutationListener())
     C.getASTMutationListener()->DeclarationMarkedUsed(this);
 
-  Used = true;
+  setIsUsed();
 }
 
 bool Decl::isReferenced() const {