DeriveAnyClass instances may skip TypeError constraints

In the presence of TypeError, one can derive instances with DeriveAnyClass that would be rejected otherwise. A simplistic example would be:

{-# LANGUAGE DeriveAnyClass       #-}
{-# LANGUAGE DataKinds            #-}
{-# LANGUAGE FlexibleContexts     #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE UndecidableSuperClasses #-}
module T

where

import GHC.TypeLits(TypeError, ErrorMessage(..))


class TypeError ('Text "BOOM") => C a where
    f :: a -> ()
    f _ = ()


data T = T
  deriving(C)

Of course, any attempt to use the instance leads to a type-error. However, the instance is rejected right away using a normal instance declaration or StandaloneDeriving.

While this is a toy example, it can actually happen when using Generics and default-signatures, where one would puts a TypeError in an instance head for one of the Generics constructors to give a better error message.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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