testsuite: Add (broken) test for #15473

testsuite/tests/typecheck/should_compile/T15473.hs

+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeOperators #-}
+-- {-# LANGUAGE UndecidableInstances #-}
+module Bug where
+
+type family Undefined :: k where {}
+
+type family LetInterleave xs t ts is (a_ahkO :: [a]) (a_ahkP :: [[a]]) :: [[a]] where
+  LetInterleave xs t ts is y z = Undefined y z
+

testsuite/tests/typecheck/should_compile/all.T

@@ -647,3 +647,4 @@ test('T15431a', normal, compile, [''])
 test('T15428', normal, compile, [''])
 test('T15412', normal, compile, [''])
 test('T15141', normal, compile, [''])
+test('T15473', expect_broken(15473), compile, [''])