1 files changed, 33 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_compile/T5051.hs b/testsuite/tests/typecheck/should_compile/T5051.hs
new file mode 100644
index 0000000000..e98c074c4b
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T5051.hs
@@ -0,0 +1,33 @@
+{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
+
+-- A very delicate interaction of overlapping instances
+
+module T5051 where
+
+data T = T deriving( Eq, Ord )
+instance Eq [T] 
+
+foo :: Ord a => [a] -> Bool
+foo x = x >= x
+
+-- Bizarrely, the defn of 'foo' failed in GHC 7.0.3 with
+-- T5051.hs:14:10:
+--    Overlapping instances for Eq [a]
+--      arising from a use of `>'
+--    Matching instances:
+--      instance Eq a => Eq [a] -- Defined in GHC.Classes
+--      instance [overlap ok] Eq [T] -- Defined at T5051.hs:9:10-15
+--    (The choice depends on the instantiation of `a'
+--     To pick the first instance above, use -XIncoherentInstances
+--     when compiling the other instance declarations)
+--    In the expression: x > x
+--
+-- Reason: the dfun for Ord [a] (in the Prelude) had a "silent"
+-- superclass parameter, thus
+--     $dfOrdList :: forall a. (Eq [a], Ord a) => Ord [a]
+-- Using the dfun means we need Eq [a], and that gives rise to the
+-- overlap error.
+--
+-- This is terribly confusing: the use of (>=) means we need Ord [a],
+-- and if we have Ord a (which we do) we should be done.
+-- A very good reason for not having silent parameters!