#12175: Instance resolution regression -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code works in 7.10, but fails in GHC 8.0.1: {{{ {-# LANGUAGE ConstraintKinds, MultiParamTypeClasses, TypeFamilies, UndecidableInstances #-} import GHC.Exts class Foo a instance (Foo a, Foo b, CTypeOf a ~ CTypeOf b) => Foo (a,b) type family TElt r :: Constraint type instance TElt r = (Foo r, Dispatch (CTypeOf r) r) type family CTypeOf x where CTypeOf (a,b) = CTypeOf a class (repr ~ CTypeOf r) => Dispatch repr r data CT r = CT [r] toCT :: (Foo r) => CT r -> CT r toCT = undefined unzipT :: (TElt a, TElt b, TElt (a,b)) => CT (a,b) -> (CT a, CT b) unzipT = unzipT . toCT main :: IO () main = undefined }}} with the errors {{{ Main.hs:1:1: error: solveWanteds: too many iterations (limit = 4) Unsolved: WC {wc_simple = [D] _ :: Dispatch fsk_a3GC b (CDictCan) [W] hole{a3Hh} :: CTypeOf a ~ CTypeOf b (CNonCanonical) [D] _ :: fsk_a3GC ~ CTypeOf b (CDictCan) [D] _ :: fsk_a3GC ~ CTypeOf b (CDictCan) [D] _ :: fsk_a3GC ~ CTypeOf b (CNonCanonical)} New superclasses found Set limit with -fconstraint-solver-iterations=n; n=0 for no limit Main.hs:22:19: error: • Couldn't match type ‘CTypeOf a’ with ‘CTypeOf b’ arising from a use of ‘toCT’ NB: ‘CTypeOf’ is a type function, and may not be injective • In the second argument of ‘(.)’, namely ‘toCT’ In the expression: unzipT . toCT In an equation for ‘unzipT’: unzipT = unzipT . toCT • Relevant bindings include unzipT :: CT (a, b) -> (CT a, CT b) (bound at Main.hs:22:1) }}} I don't understand the first error at all. The second error seems to be due to calling `toCT` on the type `CT (a,b)`, which requires the constraint `Foo (a,b)`. Rather than using the supplied constraint from `TElt (a,b)`, GHC is trying to resolve the instance provided, which requires `CType a ~ CType b`. Possibly related: #10338, #11948 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12175> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler