Test for Trac #1456

testsuite/tests/ghc-regress/typecheck/should_compile/all.T

@@ -243,6 +243,7 @@ test('tc227', normal, compile, [''])
 test('tc228', normal, compile, [''])
 test('tc229', normal, compile, [''])
 test('tc230', normal, compile, [''])
+test('tc231', normal, compile, [''])
 
 test('faxen', normal, compile, [''])
 

testsuite/tests/ghc-regress/typecheck/should_compile/tc231.hs

+{-# OPTIONS_GHC -fglasgow-exts  -ddump-types #-}
+
+-- See Trac #1456
+
+-- The key thing here is that foo should get the type
+--     foo :: forall b s t1. (Zork s (Z [Char]) b)
+--	   => Q s (Z [Char]) t1 -> ST s ()
+
+-- Note the quantification over 'b', which was previously
+-- omitted; see Note [Important subtlety in oclose] in FunDeps
+
+
+module ShouldCompile where
+
+import Control.Monad.ST
+
+data Q s a chain = Node s a chain
+
+data Z a  = Z  a
+
+s :: Q t (Z [Char]) t1 -> Q t (Z [Char]) t1
+s = undefined
+
+class  Zork s a b | a -> b where
+  huh :: Q s a chain ->  ST s ()
+
+foo b = huh (s b)
+

testsuite/tests/ghc-regress/typecheck/should_compile/tc231.stderr

+TYPE SIGNATURES
+    foo :: forall b s t1. (Zork s (Z [Char]) b) => Q s (Z [Char]) t1 -> ST s ()
+    s :: forall t t1. Q t (Z [Char]) t1 -> Q t (Z [Char]) t1
+TYPE CONSTRUCTORS
+    data Q s a chain
+	RecFlag NonRecursive
+	Generics: no
+	= Node :: forall s a chain. s -> a -> chain -> Q s a chain Stricts: _ _ _
+	FamilyInstance: none
+    data Z a
+	RecFlag NonRecursive
+	Generics: no
+	= Z :: forall a. a -> Z a Stricts: _
+	FamilyInstance: none
+Tycons with generics:
+Dependent modules: []
+Dependent packages: [base]