Add regression test for #16946

testsuite/tests/typecheck/should_compile/T16946.hs

+{-# LANGUAGE RankNTypes, TypeFamilies, PolyKinds, FunctionalDependencies #-}
+module T16946 where
+
+import Data.Kind
+
+class CatMonad (c :: k -> k -> Type) (m :: forall (x :: k) (y :: k). c x y -> Type -> Type) | c -> m where
+  type Id c :: c x x
+
+  xpure :: a -> m (Id c) a
+
+boom :: forall k (c :: k -> k -> Type) (m :: forall (x :: k) (y :: k). c x y -> Type -> Type) a. CatMonad c m => a -> m (Id c) a
+boom = xpure

testsuite/tests/typecheck/should_compile/all.T

@@ -684,3 +684,4 @@ test('UnlifNewUnify', normal, compile, [''])
 test('UnliftedNewtypesLPFamily', normal, compile, [''])
 test('UnliftedNewtypesDifficultUnification', normal, compile, [''])
 test('T16832', normal, ghci_script, ['T16832.script'])
+test('T16946', normal, compile, [''])