diff --git a/libcxx/utils/libcxx/test/dsl.py b/libcxx/utils/libcxx/test/dsl.py
--- a/libcxx/utils/libcxx/test/dsl.py
+++ b/libcxx/utils/libcxx/test/dsl.py
@@ -222,12 +222,16 @@
     self._compileFlag = compileFlag
     self._linkFlag = linkFlag
     self._isSupported = when
+    self.__cachedIsSupported = None
 
   def isSupported(self, config):
     """
     Return whether the feature is supported by the given TestingConfig.
     """
-    return self._isSupported(config)
+    if self.__cachedIsSupported is None:
+      self.__cachedIsSupported = self._isSupported(config)
+      assert self.__cachedIsSupported is not None, "Should return True or False"
+    return self.__cachedIsSupported
 
   def enableIn(self, config):
     """