ConstrainedClassMethods makes GeneralizedNewtypeDeriving fail

Example:

{-# language GeneralizedNewtypeDeriving #-}
{-# language ConstrainedClassMethods #-}

instance ToDoc Int where toDoc = undefined

newtype P = P Int deriving ( ToDoc )

type Doc = ()

class ToDoc a where 
    toDoc :: ToDoc a => a -> Doc

compile with ghc-8.0.2:

C.hs:6:30: error:
    • Couldn't match type ‘Int’ with ‘P’
        arising from the coercion of the method ‘toDoc’
          from type ‘ToDoc Int => Int -> Doc’ to type ‘ToDoc P => P -> Doc’
    • When deriving the instance for (ToDoc P)

With ghc-8.0.1, it is fine.

When I drop the constraint on the method declaration, ghc-8.0.2 compiles the program.

This repetition of the class constraint in the method declaration actually came from a copy-paste error, so this issue does not affect my code, but it's still puzzling. Repeating the class constraint should not change the meaning of the program?

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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