diff --git a/clang/include/clang/AST/ExternalASTSource.h b/clang/include/clang/AST/ExternalASTSource.h
--- a/clang/include/clang/AST/ExternalASTSource.h
+++ b/clang/include/clang/AST/ExternalASTSource.h
@@ -66,9 +66,8 @@
   /// whenever we might have added new redeclarations for existing decls.
   uint32_t CurrentGeneration = 0;
 
-  /// Whether this AST source also provides information for
-  /// semantic analysis.
-  bool SemaSource = false;
+  /// LLVM-style RTTI.
+  static char ID;
 
 public:
   ExternalASTSource() = default;
@@ -325,6 +324,12 @@
 
   virtual void getMemoryBufferSizes(MemoryBufferSizes &sizes) const;
 
+  /// LLVM-style RTTI.
+  /// \{
+  virtual bool isA(const void *ClassID) const { return ClassID == &ID; }
+  static bool classof(const ExternalASTSource *S) { return S->isA(&ID); }
+  /// \}
+
 protected:
   static DeclContextLookupResult
   SetExternalVisibleDeclsForName(const DeclContext *DC,
diff --git a/clang/include/clang/Sema/ExternalSemaSource.h b/clang/include/clang/Sema/ExternalSemaSource.h
--- a/clang/include/clang/Sema/ExternalSemaSource.h
+++ b/clang/include/clang/Sema/ExternalSemaSource.h
@@ -50,10 +50,11 @@
 /// external AST sources that also provide information for semantic
 /// analysis.
 class ExternalSemaSource : public ExternalASTSource {
+  /// LLVM-style RTTI.
+  static char ID;
+
 public:
-  ExternalSemaSource() {
-    ExternalASTSource::SemaSource = true;
-  }
+  ExternalSemaSource() = default;
 
   ~ExternalSemaSource() override;
 
@@ -222,10 +223,13 @@
     return false;
   }
 
-  // isa/cast/dyn_cast support
-  static bool classof(const ExternalASTSource *Source) {
-    return Source->SemaSource;
+  /// LLVM-style RTTI.
+  /// \{
+  bool isA(const void *ClassID) const override {
+    return ClassID == &ID || ExternalASTSource::isA(ClassID);
   }
+  static bool classof(const ExternalASTSource *S) { return S->isA(&ID); }
+  /// \}
 };
 
 } // end namespace clang
diff --git a/clang/include/clang/Sema/MultiplexExternalSemaSource.h b/clang/include/clang/Sema/MultiplexExternalSemaSource.h
--- a/clang/include/clang/Sema/MultiplexExternalSemaSource.h
+++ b/clang/include/clang/Sema/MultiplexExternalSemaSource.h
@@ -36,6 +36,8 @@
 /// external AST sources that also provide information for semantic
 /// analysis.
 class MultiplexExternalSemaSource : public ExternalSemaSource {
+  /// LLVM-style RTTI.
+  static char ID;
 
 private:
   SmallVector<ExternalSemaSource *, 2> Sources; // doesn't own them.
@@ -352,9 +354,13 @@
   bool MaybeDiagnoseMissingCompleteType(SourceLocation Loc,
                                         QualType T) override;
 
-  // isa/cast/dyn_cast support
-  static bool classof(const MultiplexExternalSemaSource*) { return true; }
-  //static bool classof(const ExternalSemaSource*) { return true; }
+  /// LLVM-style RTTI.
+  /// \{
+  bool isA(const void *ClassID) const override {
+    return ClassID == &ID || ExternalSemaSource::isA(ClassID);
+  }
+  static bool classof(const ExternalASTSource *S) { return S->isA(&ID); }
+  /// \}
 };
 
 } // end namespace clang
diff --git a/clang/lib/AST/ExternalASTSource.cpp b/clang/lib/AST/ExternalASTSource.cpp
--- a/clang/lib/AST/ExternalASTSource.cpp
+++ b/clang/lib/AST/ExternalASTSource.cpp
@@ -24,6 +24,8 @@
 
 using namespace clang;
 
+char ExternalASTSource::ID;
+
 ExternalASTSource::~ExternalASTSource() = default;
 
 llvm::Optional<ExternalASTSource::ASTSourceDescriptor>
diff --git a/clang/lib/Sema/MultiplexExternalSemaSource.cpp b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
--- a/clang/lib/Sema/MultiplexExternalSemaSource.cpp
+++ b/clang/lib/Sema/MultiplexExternalSemaSource.cpp
@@ -15,6 +15,8 @@
 
 using namespace clang;
 
+char MultiplexExternalSemaSource::ID;
+
 ///Constructs a new multiplexing external sema source and appends the
 /// given element to it.
 ///
diff --git a/clang/lib/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -1917,6 +1917,7 @@
 
 // Pin this vtable to this file.
 ExternalSemaSource::~ExternalSemaSource() {}
+char ExternalSemaSource::ID;
 
 void ExternalSemaSource::ReadMethodPool(Selector Sel) { }
 void ExternalSemaSource::updateOutOfDateSelector(Selector Sel) { }