Index: include/clang/AST/DeclBase.h
===================================================================
--- include/clang/AST/DeclBase.h
+++ include/clang/AST/DeclBase.h
@@ -528,7 +528,7 @@
   /// \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
@@ -329,25 +329,27 @@
   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;
     }
 
Index: test/Modules/Inputs/PR27401/a.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27401/a.h
@@ -0,0 +1,17 @@
+#ifndef _LIBCPP_ALGORITHM
+#define _LIBCPP_ALGORITHM
+template <class _Tp, _Tp>
+struct integral_constant {
+  static const _Tp value = _Tp();
+};
+
+template <class _Tp>
+struct is_nothrow_default_constructible
+	: integral_constant<bool, __is_constructible(_Tp)> {};
+
+template <class _Tp>
+struct is_nothrow_move_constructible
+    : integral_constant<bool, __is_constructible(_Tp, _Tp)> {};
+
+class allocator {};
+#endif
Index: test/Modules/Inputs/PR27401/b.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27401/b.h
@@ -0,0 +1,21 @@
+#include "a.h"
+#ifndef _LIBCPP_VECTOR
+template <class, class _Allocator>
+class __vector_base {
+protected:
+  _Allocator __alloc() const;
+  __vector_base(_Allocator);
+};
+
+template <class _Tp, class _Allocator = allocator>
+class vector : __vector_base<_Tp, _Allocator> {
+public:
+  vector() noexcept(is_nothrow_default_constructible<_Allocator>::value);
+  vector(const vector &);
+  vector(vector &&)
+      noexcept(is_nothrow_move_constructible<_Allocator>::value);
+};
+
+#endif
+void GetUniquePtrType() { vector<char> v; }
+
Index: test/Modules/Inputs/PR27401/module.modulemap
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27401/module.modulemap
@@ -0,0 +1 @@
+module "b" { header "b.h" export * }
Index: test/Modules/pr27401.cpp
===================================================================
--- /dev/null
+++ test/Modules/pr27401.cpp
@@ -0,0 +1,38 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -std=c++11 -I%S/Inputs/PR27401 -verify %s
+// RUN: %clang_cc1 -std=c++11 -fmodules -fmodule-map-file=%S/Inputs/PR27401/module.modulemap -fmodules-cache-path=%t -I%S/Inputs/PR27401 -verify %s
+
+#include "a.h"
+#define _LIBCPP_VECTOR
+template <class, class _Allocator>
+class __vector_base {
+protected:
+  _Allocator __alloc() const;
+  __vector_base(_Allocator);
+};
+
+template <class _Tp, class _Allocator = allocator>
+class vector : __vector_base<_Tp, _Allocator> {
+public:
+  vector() noexcept(is_nothrow_default_constructible<_Allocator>::value);
+  vector(const vector &);
+  vector(vector &&)
+      noexcept(is_nothrow_move_constructible<_Allocator>::value);
+};
+
+template <class _Tp, class _Allocator>
+vector<_Tp, _Allocator>::vector(const vector &__x) : __vector_base<_Tp, _Allocator>(__x.__alloc()) {}
+
+  struct CommentOptions {
+    vector<char>  ParseAllComments;
+    CommentOptions() {}
+  };
+  struct PrintingPolicy {
+    PrintingPolicy(CommentOptions LO) : LangOpts(LO) {}
+    CommentOptions LangOpts;
+  };
+
+#include "b.h"
+CommentOptions fn1() { return fn1(); }
+
+// expected-no-diagnostics