Test Trac bug #795

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

@@ -213,6 +213,7 @@ test('tc200', normal, compile, [''])
 test('tc201', normal, compile, [''])
 test('tc202', normal, compile, [''])
 test('tc203', normal, compile, [''])
+test('tc204', normal, compile, [''])
 
 # Omitting temporarily
 # test('syn-perf', normal, compile, ['-fcontext-stack=30'])

testsuite/tests/ghc-regress/typecheck/should_compile/tc204.hs

+{-# OPTIONS -fglasgow-exts -dcore-lint #-}
+
+-- The dict-bindings attached to an IPBinds 
+-- need not be in recursive order.  This is
+-- a long-standing bug, which lasted up to 
+-- and including GHC 6.4.2
+
+module Bug795(foo) where
+
+data Arg = E Integer | T Bool deriving (Eq, Show)
+
+foo :: Integer -> [Arg] -> IO String
+foo 1 as = do { let ?err = "my custom error"
+               ; let ws = (show (firstE as))
+               ; return (show (firstE as)) }
+
+firstE :: (?err :: String) => [Arg] -> Integer
+firstE = error "urk"