Test for Trac #2238

tests/ghc-regress/indexed-types/should_compile/T2238.hs

@@ -0,0 +1,39 @@
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE EmptyDataDecls #-}
+{-# LANGUAGE FunctionalDependencies #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+
+-- Trac #2238
+-- Notice that class CTF has just one value field, but
+-- it also has an equality predicate. 
+-- See Note [Class newtypes and equality predicates] in BuildTyCl
+
+module Foo where
+
+data A
+data B
+
+-- via functional dependencies
+
+class HowFD a how | a -> how
+
+class HowFD a how => CFD a how where
+  cfd :: a -> String
+  cfd _ = "cfd"
+instance HowFD a how => CFD a how
+
+instance HowFD Bool A
+
+-- via type families
+
+type family HowTF a
+
+class how ~ HowTF a => CTF a how where
+  ctf :: a -> String
+  ctf _ = "ctf"
+
+instance how ~ HowTF a => CTF a how
+
+type instance HowTF Bool = A

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

@@ -106,3 +106,4 @@ test('GivenCheckTop', normal, compile, [''])
 
 test('Gentle', normal, compile, [''])
 test('T1981', normal, compile, [''])
+test('T2238', normal, compile, [''])