#8391: FunDeps coverage condition check failure with PolyKinds ----------------------------+---------------------------------------------- Reporter: sbarclay | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects valid program Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: #8356 | ----------------------------+---------------------------------------------- The following code: {{{ #!haskell {-# LANGUAGE FunctionalDependencies, FlexibleInstances, UndecidableInstances, TypeFamilies, PolyKinds #-} module FunDepsIssue where type Foo a = a class Foo a ~ b => Bar a b | a -> b instance Foo a ~ b => Bar a b }}} results in: {{{ Illegal instance declaration for ‛Bar k a b’ Multiple uses of this instance may be inconsistent with the functional dependencies of the class. In the instance declaration for ‛Bar a b’ }}} This appears to be a regression from 7.6.3, where it compiles OK. The same error occurs when the type synonym is replaced with a type family (which is the case I was originally interested in) or a type constructor like `Data.Proxy`, as long as it's poly-kinded it seems. So the following cases also fail to compile: {{{ #!haskell type family Foo (a :: k) :: * type family Foo (a :: *) :: k type family Foo (a :: k) :: k type family Foo (a :: k) :: k' }}} Looking through the source code, I think that the issue is that the coverage condition check doesn't close over kind variables, when it compares the vars on each side of the functional dependency. But `Foo a` has a hidden kind variable, so the l.h. varset `{a}` is insufficient to fix `b` using `Foo a ~ b`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8391 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler