Test Trac #3484

testsuite/tests/ghc-regress/indexed-types/should_compile/T3484.hs

+{-# LANGUAGE GADTs, RankNTypes, TypeFamilies, FlexibleContexts, ScopedTypeVariables #-}
+{-# OPTIONS_GHC -Wall #-}
+module Absurd where
+
+data Z = Z
+newtype S n = S n
+class Nat n where
+   caseNat :: (n ~ Z => r) -> (forall p. (n ~ S p, Nat p) => p -> r) -> n -> r
+instance Nat Z where
+   caseNat = error "urk1"
+instance Nat n => Nat (S n) where
+   caseNat = error "urk2"
+
+-- empty type
+newtype Naught = Naught (forall a. a)
+-- types are equal!
+data TEq a b where
+   TEq :: (a ~ b) => TEq a b
+
+type family NatEqProves m n
+type instance NatEqProves (S m) (S n) = TEq m n
+
+noConf :: (Nat m, Nat n) => m -> TEq m n -> NatEqProves m n
+noConf = undefined
+predEq :: TEq (S a) (S b) -> TEq a b
+predEq = undefined
+
+data IsEq a b = Yes (TEq a b) | No (TEq a b -> Naught)
+
+natEqDec :: forall m n. (Nat m, Nat n) => m -> n -> IsEq m n
+natEqDec m n = caseNat undefined mIsS m where
+    mIsS :: forall pm. (m ~ S pm, Nat pm) => pm -> IsEq m n
+    mIsS pm = caseNat undefined nIsS n where
+        nIsS :: forall pn. (n ~ S pn, Nat pn) => pn -> IsEq m n
+        nIsS pn = case natEqDec pm pn of
+            Yes TEq -> Yes TEq
+            No contr -> No (contr . noConf m)
+--            No contr -> No (contr . predEq)
+
+-- strange things:
+-- (1) commenting out the "Yes" case or changing it to "undefined" makes compilation succeed
+-- (2) replacing the "No" line with with the commented out "No" line makes compilation succeed
\ No newline at end of file

testsuite/tests/ghc-regress/indexed-types/should_compile/all.T

@@ -162,3 +162,4 @@ test('T4494', normal, compile, [''])
 test('DataFamDeriv', normal, compile, [''])
 test('T1769', if_compiler_lt('ghc', '7.1', expect_fail), compile, [''])
 test('T4497', normal, compile, [''])
+test('T3484', normal, compile, [''])