Index: include/clang/AST/DeclBase.h
===================================================================
--- include/clang/AST/DeclBase.h
+++ include/clang/AST/DeclBase.h
@@ -529,7 +529,7 @@
   /// \brief Set whether the declaration is used, in the sense of odr-use.
   ///
   /// This should only be used immediately after creating a declaration.
-  void setIsUsed() { Used = true; }
+  void setIsUsed() { getCanonicalDecl()->Used = true; }
 
   /// \brief Mark the declaration used, in the sense of odr-use.
   ///
Index: lib/AST/DeclBase.cpp
===================================================================
--- lib/AST/DeclBase.cpp
+++ lib/AST/DeclBase.cpp
@@ -340,25 +340,27 @@
   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>())
+  if (CheckUsedAttr && canonD->hasAttr<UsedAttr>())
     return true;
 
-  return false; 
+  return false;
 }
 
 void Decl::markUsed(ASTContext &C) {
-  if (Used)
+  if (isUsed())
     return;
 
+  // Q: Do we need to call the callback with the canonical decl?
   if (C.getASTMutationListener())
     C.getASTMutationListener()->DeclarationMarkedUsed(this);
 
-  Used = true;
+  getCanonicalDecl()->Used = true;
 }
 
 bool Decl::isReferenced() const { 
Index: lib/Sema/SemaExpr.cpp
===================================================================
--- lib/Sema/SemaExpr.cpp
+++ lib/Sema/SemaExpr.cpp
@@ -13011,17 +13011,9 @@
       UndefinedButUsed.insert(std::make_pair(Func->getCanonicalDecl(), Loc));
   }
 
-  // Normally the most current decl is marked used while processing the use and
-  // any subsequent decls are marked used by decl merging. This fails with
-  // template instantiation since marking can happen at the end of the file
-  // and, because of the two phase lookup, this function is called with at
-  // decl in the middle of a decl chain. We loop to maintain the invariant
-  // that once a decl is used, all decls after it are also used.
-  for (FunctionDecl *F = Func->getMostRecentDecl();; F = F->getPreviousDecl()) {
-    F->markUsed(Context);
-    if (F == Func)
-      break;
-  }
+  // It is sufficient for the canonical decl to be marked as used.
+  Func->markUsed(Context);
+
 }
 
 static void
Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -3112,11 +3112,6 @@
       Previous->IdentifierNamespace &
       (Decl::IDNS_Ordinary | Decl::IDNS_Tag | Decl::IDNS_Type);
 
-  // If the previous declaration is marked as used, then this declaration should
-  // be too.
-  if (Previous->Used)
-    D->Used = true;
-
   // If the declaration declares a template, it may inherit default arguments
   // from the previous declaration.
   if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D))
@@ -3865,7 +3860,7 @@
       // ASTMutationListeners other than an ASTWriter.
 
       // Maintain AST consistency: any later redeclarations are used too.
-      forAllLaterRedecls(D, [](Decl *D) { D->Used = true; });
+      D->setIsUsed();
       break;
     }