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diff --git a/testsuite/tests/polykinds/CuskFam.hs b/testsuite/tests/polykinds/CuskFam.hs
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+++ b/testsuite/tests/polykinds/CuskFam.hs
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+{-# LANGUAGE TypeFamilies, PolyKinds, DataKinds #-}
+-- {-# LANGUAGE CUSKs #-}    -- enabled by default
+
+module CuskFam where
+
+type family F a   -- non-injective
+
+type family X :: F a
+    -- Used to fail with:
+    --
+    --    • Couldn't match expected kind ‘F a1’ with actual kind ‘F a’
+    --      NB: ‘F’ is a non-injective type family
+    --      The type variable ‘a1’ is ambiguous
+    --    • In the type family declaration for ‘X’
+    --
+    -- See Note [Unifying implicit CUSK variables] in TcHsType