diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
index 638895ee4dc677..ad1150b6251b25 100644
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -124,7 +124,7 @@ class function_ref<Ret(Params...)> {
     return callback(callable, std::forward<Params>(params)...);
   }
 
-  operator bool() const { return callback; }
+  explicit operator bool() const { return callback; }
 };
 
 // deleter - Very very very simple method that is used to invoke operator