diff --git a/clang/include/clang/AST/ExternalASTSource.h b/clang/include/clang/AST/ExternalASTSource.h
index 6db233641220e751c6987c262c15fb2af2c51a92..846813adf7c34e137eaa59b65b7324aa81220d0a 100644
--- a/clang/include/clang/AST/ExternalASTSource.h
+++ b/clang/include/clang/AST/ExternalASTSource.h
@@ -211,7 +211,7 @@ public:
     return sizes;
   }
 
-  virtual void getMemoryBufferSizes(MemoryBufferSizes &sizes) const = 0;
+  virtual void getMemoryBufferSizes(MemoryBufferSizes &sizes) const;
 
 protected:
   static DeclContextLookupResult
diff --git a/clang/lib/AST/ExternalASTSource.cpp b/clang/lib/AST/ExternalASTSource.cpp
index 89bf56db1af73c3d19337db88061e49c2113e682..f428318a21e329795ca5c79aa0bbfc819ab7b06f 100644
--- a/clang/lib/AST/ExternalASTSource.cpp
+++ b/clang/lib/AST/ExternalASTSource.cpp
@@ -57,3 +57,5 @@ ExternalASTSource::FindExternalLexicalDecls(const DeclContext *DC,
                                          llvm::SmallVectorImpl<Decl*> &Result) {
   return true;
 }
+
+void ExternalASTSource::getMemoryBufferSizes(MemoryBufferSizes &sizes) const { }