Add a testcase for #11362

This reproduces the issue that I encountered in #11362.

Test Plan: this testcase

Reviewers: simonpj, bgamari, austin

Reviewed By: simonpj

Subscribers: thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D1917

GHC Trac Issues: #11362

testsuite/tests/polykinds/T11362.hs

+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE GADTs #-}
+
+module T11362 where
+-- this file when compiled with -dunique-increment=-1 made GHC crash
+
+data Sum a b = L a | R b
+
+data Sum1 (a :: k1 -> *) (b :: k2 -> *) :: Sum k1 k2 -> * where
+  LL :: a i -> Sum1 a b (L i)
+  RR :: b i -> Sum1 a b (R i)
+
+data Code i o = F (Code (Sum i o) o)
+
+-- An interpretation for `Code` using a data family works:
+data family In (f :: Code i o) :: (i -> *) -> (o -> *)
+
+data instance In (F f) r o where
+  MkIn :: In f (Sum1 r (In (F f) r)) o -> In (F f) r o
+
+-- Requires polymorphic recursion
+data In' (f :: Code i o) :: (i -> *) -> o -> * where
+  MkIn' :: In' g (Sum1 r (In' (F g) r)) t -> In' (F g) r t

testsuite/tests/polykinds/all.T

@@ -139,4 +139,6 @@ test('T11480b', normal, compile, [''])
 test('T11523', normal, compile, [''])
 test('T11520', normal, compile_fail, [''])
 test('T11516', normal, compile_fail, [''])
+test('T11362', normal, compile, ['-dunique-increment=-1'])
+  # -dunique-increment=-1 doesn't work inside the file
 test('T11399', normal, compile_fail, [''])