Index: include/clang/Serialization/ASTReader.h
===================================================================
--- include/clang/Serialization/ASTReader.h
+++ include/clang/Serialization/ASTReader.h
@@ -527,11 +527,9 @@
   /// to apply once we finish processing an import.
   llvm::SmallVector<PendingUpdateRecord, 16> PendingUpdateRecords;
 
-  enum class PendingFakeDefinitionKind { NotFake, Fake, FakeLoaded };
-
   /// The DefinitionData pointers that we faked up for class definitions
   /// that we needed but hadn't loaded yet.
-  llvm::DenseMap<void *, PendingFakeDefinitionKind> PendingFakeDefinitionData;
+  llvm::DenseSet<void *> PendingFakeDefinitionData;
 
   /// Exception specification updates that have been loaded but not yet
   /// propagated across the relevant redeclaration chain. The map key is the
Index: lib/Serialization/ASTReaderDecl.cpp
===================================================================
--- lib/Serialization/ASTReaderDecl.cpp
+++ lib/Serialization/ASTReaderDecl.cpp
@@ -1757,12 +1757,14 @@
   }
 
   auto PFDI = Reader.PendingFakeDefinitionData.find(&DD);
-  if (PFDI != Reader.PendingFakeDefinitionData.end() &&
-      PFDI->second == ASTReader::PendingFakeDefinitionKind::Fake) {
+  if (PFDI != Reader.PendingFakeDefinitionData.end()) {
     // We faked up this definition data because we found a class for which we'd
     // not yet loaded the definition. Replace it with the real thing now.
+
+    Reader.PendingFakeDefinitionData.erase(PFDI);
+
+    // FIXME: handle serialized lambdas
     assert(!DD.IsLambda && !MergeDD.IsLambda && "faked up lambda definition?");
-    PFDI->second = ASTReader::PendingFakeDefinitionKind::FakeLoaded;
 
     // Don't change which declaration is the definition; that is required
     // to be invariant once we select it.
@@ -3077,8 +3079,7 @@
       RD->getCanonicalDecl()->DefinitionData = DD;
 
       // Track that we did this horrible thing so that we can fix it later.
-      Reader.PendingFakeDefinitionData.insert(
-          std::make_pair(DD, ASTReader::PendingFakeDefinitionKind::Fake));
+      Reader.PendingFakeDefinitionData.insert(DD);
     }
 
     return DD->Definition;