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
@@ -6,6 +6,7 @@
 #
 #===----------------------------------------------------------------------===##
 
+import copy
 import os
 import pipes
 import platform
@@ -37,7 +38,10 @@
         result = next(res for (key, res) in cache if key == cacheKey)
       except StopIteration: # This wasn't in the cache
         result = function(*args, **kwargs)
-        cache.append((cacheKey, result))
+        # Note: we have to use copy.deepcopy() here to avoid storing a
+        # mutable list in the cache, since that would cause cache hits
+        # even when the list changed compared to when the value was cached.
+        cache.append((copy.deepcopy(cacheKey), result))
       return result
     return f
   return decorator