Regression in superclass checking of instances
See master ticket #22912 (closed)
Summary
GHC is rejecting some instances of classes involving type families in their superclasses, that it previously accepted.
Steps to reproduce
Minimized example:
{# LANGUAGE TypeFamilies,
TypeOperators, DataKinds, ConstraintKinds, UndecidableSuperClasses, UndecidableInstances,
FlexibleInstances, MultiParamTypeClasses, FlexibleContexts, AllowAmbiguousTypes
#}
module R where
import Data.Proxy (Proxy(..))
import Data.Kind (Type)
import Data.Type.Bool
class Functor (If y Maybe Maybe) => C (y :: Bool) where
p :: Proxy y
instance (S yf yg, y ~ (yf  yg)) => C y where
p = Proxy
class Functor (If (yf  yg) Maybe Maybe) => S yf yg where
pS :: Proxy '(yf, yg)
Expected (and actual on GHC 9.0 and prior): Should compile.
Actual (GHC 9.2.1): Error message below.
R.hs:15:10: error:
• Could not deduce (Functor (If y Maybe Maybe))
arising from the superclasses of an instance declaration
from the context: (S yf yg, y ~ (yf  yg))
bound by the instance declaration at local/r.hs:15:1041
• In the instance declaration for ‘C y’

15  instance (S yf yg, y ~ (yf  yg)) => C y where
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Environment
 GHC version used: 9.2.1
EDIT (by @rae):
Despite this being labeled a regression, it turns out that older GHCs were wrong in accepting programs like this in the first place. See #20666 (comment 393745) for a runtime loop possible in GHC 8.10.7.