diff --git a/llvm/include/llvm/ADT/MapVector.h b/llvm/include/llvm/ADT/MapVector.h
--- a/llvm/include/llvm/ADT/MapVector.h
+++ b/llvm/include/llvm/ADT/MapVector.h
@@ -43,6 +43,7 @@
       "The mapped_type of the specified Map must be an integral type");
 
 public:
+  using key_type = KeyT;
   using value_type = typename VectorType::value_type;
   using size_type = typename VectorType::size_type;