diff --git a/clang/lib/AST/ExternalASTSource.cpp b/clang/lib/AST/ExternalASTSource.cpp
index 730842a..1c82c35 100644
--- a/clang/lib/AST/ExternalASTSource.cpp
+++ b/clang/lib/AST/ExternalASTSource.cpp
@@ -22,6 +22,16 @@
 
 ExternalASTSource::~ExternalASTSource() { }
 
+llvm::Optional<ExternalASTSource::ASTSourceDescriptor>
+ExternalASTSource::getSourceDescriptor(unsigned ID) {
+  return None;
+}
+
+ExternalASTSource::ASTSourceDescriptor
+ExternalASTSource::getSourceDescriptor(const Module &M) {
+  return ASTSourceDescriptor();
+}
+
 void ExternalASTSource::FindFileRegionDecls(FileID File, unsigned Offset,
                                             unsigned Length,
                                             SmallVectorImpl<Decl *> &Decls) {}