Index: include/clang/AST/DeclCXX.h
===================================================================
--- include/clang/AST/DeclCXX.h
+++ include/clang/AST/DeclCXX.h
@@ -257,30 +257,6 @@
   TypeSourceInfo *getTypeSourceInfo() const { return BaseTypeInfo; }
 };
 
-/// \brief A lazy pointer to the definition data for a declaration.
-/// FIXME: This is a little CXXRecordDecl-specific that the moment.
-template<typename Decl, typename T> class LazyDefinitionDataPtr {
-  llvm::PointerUnion<T *, Decl *> DataOrCanonicalDecl;
-
-  LazyDefinitionDataPtr update() {
-    if (Decl *Canon = DataOrCanonicalDecl.template dyn_cast<Decl*>()) {
-      if (Canon->isCanonicalDecl())
-        Canon->getMostRecentDecl();
-      else
-        // Declaration isn't canonical any more;
-        // update it and perform path compression.
-        *this = Canon->getPreviousDecl()->DefinitionData.update();
-    }
-    return *this;
-  }
-
-public:
-  LazyDefinitionDataPtr(Decl *Canon) : DataOrCanonicalDecl(Canon) {}
-  LazyDefinitionDataPtr(T *Data) : DataOrCanonicalDecl(Data) {}
-  T *getNotUpdated() { return DataOrCanonicalDecl.template dyn_cast<T*>(); }
-  T *get() { return update().getNotUpdated(); }
-};
-
 /// \brief Represents a C++ struct/union/class.
 class CXXRecordDecl : public RecordDecl {
 
@@ -543,11 +519,7 @@
     CXXBaseSpecifier *getVBasesSlowCase() const;
   };
 
-  typedef LazyDefinitionDataPtr<CXXRecordDecl, struct DefinitionData>
-      DefinitionDataPtr;
-  friend class LazyDefinitionDataPtr<CXXRecordDecl, struct DefinitionData>;
-
-  mutable DefinitionDataPtr DefinitionData;
+  mutable DefinitionData* DefinitionData;
 
   /// \brief Describes a C++ closure type (generated by a lambda expression).
   struct LambdaDefinitionData : public DefinitionData {
@@ -611,7 +583,8 @@
   };
 
   struct DefinitionData &data() const {
-    auto *DD = DefinitionData.get();
+    // Complete the redecl chain reading all necessary information.
+    auto *DD = getMostRecentDecl()->DefinitionData;
     assert(DD && "queried property of class with no definition");
     return *DD;
   }
@@ -619,7 +592,7 @@
   struct LambdaDefinitionData &getLambdaData() const {
     // No update required: a merged definition cannot change any lambda
     // properties.
-    auto *DD = DefinitionData.getNotUpdated();
+    auto *DD = DefinitionData;
     assert(DD && DD->IsLambda && "queried lambda property of non-lambda class");
     return static_cast<LambdaDefinitionData&>(*DD);
   }
@@ -696,11 +669,11 @@
   }
 
   CXXRecordDecl *getDefinition() const {
-    auto *DD = DefinitionData.get();
+    auto *DD = DefinitionData;
     return DD ? DD->Definition : nullptr;
   }
 
-  bool hasDefinition() const { return DefinitionData.get(); }
+  bool hasDefinition() const { return DefinitionData; }
 
   static CXXRecordDecl *Create(const ASTContext &C, TagKind TK, DeclContext *DC,
                                SourceLocation StartLoc, SourceLocation IdLoc,
@@ -1044,7 +1017,7 @@
   /// \brief Determine whether this class describes a lambda function object.
   bool isLambda() const {
     // An update record can't turn a non-lambda into a lambda.
-    auto *DD = DefinitionData.getNotUpdated();
+    auto *DD = DefinitionData;
     return DD && DD->IsLambda;
   }
 
Index: lib/AST/DeclCXX.cpp
===================================================================
--- lib/AST/DeclCXX.cpp
+++ lib/AST/DeclCXX.cpp
@@ -88,7 +88,7 @@
                              CXXRecordDecl *PrevDecl)
     : RecordDecl(K, TK, C, DC, StartLoc, IdLoc, Id, PrevDecl),
       DefinitionData(PrevDecl ? PrevDecl->DefinitionData
-                              : DefinitionDataPtr(this)),
+                              : nullptr),
       TemplateOrInstantiation() {}
 
 CXXRecordDecl *CXXRecordDecl::Create(const ASTContext &C, TagKind TK,
Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -1540,9 +1540,9 @@
 
 void ASTDeclReader::MergeDefinitionData(
     CXXRecordDecl *D, struct CXXRecordDecl::DefinitionData &&MergeDD) {
-  assert(D->DefinitionData.getNotUpdated() &&
+  assert(D->DefinitionData &&
          "merging class definition into non-definition");
-  auto &DD = *D->DefinitionData.getNotUpdated();
+  auto &DD = *D->DefinitionData;
 
   if (DD.Definition != MergeDD.Definition) {
     // Track that we merged the definitions.
@@ -1665,7 +1665,7 @@
   // because we're reading an update record, or because we've already done some
   // merging. Either way, just merge into it.
   CXXRecordDecl *Canon = D->getCanonicalDecl();
-  if (Canon->DefinitionData.getNotUpdated()) {
+  if (Canon->DefinitionData) {
     MergeDefinitionData(Canon, std::move(*DD));
     D->DefinitionData = Canon->DefinitionData;
     return;
@@ -2001,8 +2001,8 @@
 
         // This declaration might be a definition. Merge with any existing
         // definition.
-        if (auto *DDD = D->DefinitionData.getNotUpdated()) {
-          if (CanonSpec->DefinitionData.getNotUpdated())
+        if (auto *DDD = D->DefinitionData) {
+          if (CanonSpec->DefinitionData)
             MergeDefinitionData(CanonSpec, std::move(*DDD));
           else
             CanonSpec->DefinitionData = D->DefinitionData;
@@ -2326,8 +2326,8 @@
     // FIXME: This is duplicated in several places. Refactor.
     auto *ExistingClass =
         cast<CXXRecordDecl>(ExistingPattern)->getCanonicalDecl();
-    if (auto *DDD = DClass->DefinitionData.getNotUpdated()) {
-      if (ExistingClass->DefinitionData.getNotUpdated()) {
+    if (auto *DDD = DClass->DefinitionData) {
+      if (ExistingClass->DefinitionData) {
         MergeDefinitionData(ExistingClass, std::move(*DDD));
       } else {
         ExistingClass->DefinitionData = DClass->DefinitionData;
@@ -2765,9 +2765,9 @@
 
   if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(DC)) {
     // Try to dig out the definition.
-    auto *DD = RD->DefinitionData.getNotUpdated();
+    auto *DD = RD->DefinitionData;
     if (!DD)
-      DD = RD->getCanonicalDecl()->DefinitionData.getNotUpdated();
+      DD = RD->getCanonicalDecl()->DefinitionData;
 
     // If there's no definition yet, then DC's definition is added by an update
     // record, but we've not yet loaded that update record. In this case, we
@@ -3772,7 +3772,7 @@
 
     case UPD_CXX_INSTANTIATED_CLASS_DEFINITION: {
       auto *RD = cast<CXXRecordDecl>(D);
-      auto *OldDD = RD->getCanonicalDecl()->DefinitionData.getNotUpdated();
+      auto *OldDD = RD->getCanonicalDecl()->DefinitionData;
       bool HadRealDefinition =
           OldDD && (OldDD->Definition != RD ||
                     !Reader.PendingFakeDefinitionData.count(OldDD));
Index: test/Modules/Inputs/PR27754/RConversionRuleParser.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27754/RConversionRuleParser.h
@@ -0,0 +1,7 @@
+#define _STL_ALGOBASE_H
+template < typename _Tp, _Tp > struct integral_constant
+{
+};
+typedef integral_constant < bool, true > true_type;
+#include <map>
+#include "TSchemaType.h"
Index: test/Modules/Inputs/PR27754/TMetaUtils.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27754/TMetaUtils.h
@@ -0,0 +1,2 @@
+#include "RConversionRuleParser.h"
+void fn1() { true_type(); }
Index: test/Modules/Inputs/PR27754/TSchemaType.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27754/TSchemaType.h
@@ -0,0 +1,4 @@
+#ifndef _STL_ALGOBASE_H
+template <typename _Tp, _Tp> struct integral_constant {};
+struct A : integral_constant<bool, true> {};
+#endif
Index: test/Modules/Inputs/PR27754/map
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27754/map
@@ -0,0 +1,3 @@
+class _Rb_tree {
+  _Rb_tree() { true_type(); }
+};
Index: test/Modules/Inputs/PR27754/module.modulemap
===================================================================
--- /dev/null
+++ test/Modules/Inputs/PR27754/module.modulemap
@@ -0,0 +1,3 @@
+module "RConversionRuleParser.h" { header "RConversionRuleParser.h" }
+module "TMetaUtils.h" { header "TMetaUtils.h" }
+module "TSchemaType.h" { header "TSchemaType.h" }
Index: test/Modules/pr27754.cpp
===================================================================
--- /dev/null
+++ test/Modules/pr27754.cpp
@@ -0,0 +1,7 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -std=c++11 -I%S/Inputs/PR27754 -verify %s
+// RUN: %clang_cc1 -std=c++11 -fmodules -fmodule-map-file=%S/Inputs/PR27754/module.modulemap -fmodules-cache-path=%t -I%S/Inputs/PR27754/ -verify %s
+
+#include "TMetaUtils.h"
+
+// expected-no-diagnostics