diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 38ee478c..40d206f6 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -12,10 +12,7 @@ import config : requires ;
 
 path-constant TOP : . ;
 
-# Adding -Wundef is blocked on (at least)
-# https://github.com/boostorg/type_traits/issues/165
-
-local gcc-flags = -Wsign-promo -Wconversion -Wsign-conversion -Wfloat-equal -Wshadow -Wno-variadic-macros ;
+local gcc-flags = -Wsign-promo -Wconversion -Wsign-conversion -Wfloat-equal -Wshadow -Wundef -Wno-variadic-macros ;
 local clang-flags = $(gcc-flags) -Wno-c99-extensions ;
 local msvc-flags = /wd4494 ;