#10562: GHC 7.10.2 RC cannot build boolsimplifier-0.1.8 -------------------------------------+------------------------------------- Reporter: snoyberg | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.2-rc1 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): Here is a smaller test case {{{ {-# LANGUAGE GADTs, TypeFamilies #-} module T10562 where type family Flip a data QueryRep qtyp a where QAtom :: a -> QueryRep () a QOp :: QueryRep (Flip qtyp) a -> QueryRep qtyp a instance (Eq a) => Eq (QueryRep qtyp a) instance (Ord a) => Ord (QueryRep qtyp a) where compare (QOp a) (QOp b) = a `compare` b }}} With 7.10 we get {{{ T10562.hs:13:31: Overlapping instances for Eq (QueryRep (Flip qtyp) a) arising from a use of ‘compare’ }}} The problem arises because of the "silent superclass" trick (which is happily gone from HEAD). The instance declaration that is actually checked is {{{ instance (Eq (QueryRep qtyp a), Ord a) => Ord (QueryRep qtyp a) }}} with an extra `Eq (QueryRep qtyp a)` constraint. That gets GHC confused when it tries to solve `Eq (QueryRep (Flip qtyp) a)` from a method. A workaround is to use `-XIncoherentInstances` for this module. Exactly the same failure happens with GHC 7.8.3 doesn't it? Though apparently not with 7.8.2, mysteriously. I'm sorry but it's not feasible to get HEAD's solution into 7.10 now. It works fine in HEAD, and I'll add the example as a test case, to check it stays working. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10562#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler