Hello, these two instances really should be rejected as they violate the FD of the class: we can derive `TypeEq a a True` using the first instance and `TypeEq a a False` using the second one. Unfortunately, the check that we are using to validate FDs when `UndecidableInstances` is on, is not quite correct (relevant tickets are #9210 and #10675 where there are similar examples). -Iavor On Sun, Apr 16, 2017 at 12:13 AM, Anthony Clayden < anthony_clayden@clear.net.nz> wrote:
--ghc 7.10 or 8.0.1
{-# LANGUAGE DataKinds, KindSignatures, GADTs, MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, NoOverlappingInstances #-}
class TypeEq a a' (b :: Bool) | a a' -> b
instance (b ~ True) => TypeEq a a b instance (b ~ False) => TypeEq a a' b
Those two instance heads are nearly identical, surely they overlap? And for a type-level type equality test, they must be unifiable. But GHC doesn't complain.
If I take off the FunDep, then GHC complains.
AFAICT none of those extensions imply Overlaps, but to be sure I've put NoOverlapping.
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