expressing injectivity on functional dependencies gives orphan instances warnings

Class.hs

{-# LANGUAGE DataKinds, FlexibleInstances, FunctionalDependencies, KindSignatures, MultiParamTypeClasses, TypeFamilies #-}

module Class where

class C a b | a -> b, b -> a

Lib.hs

{-# LANGUAGE DataKinds, FlexibleInstances, MultiParamTypeClasses, TypeFamilies #-}

module Lib where

import Class


newtype Local = Local ()

instance C Local ()

gives

/Users/dr/workspace/broken-ophans-ghc/src/Lib.hs:10:1: warning: [-Worphans]
    Orphan instance: instance C Local ()
    To avoid this
        move the instance declaration to the module of the class or of the type, or
        wrap the type with a newtype and declare the instance on the new type.

It seems weird to me and it is either a bug in the orphan instances checker or there is some fundamental aspect which i don't undestand. I've been able to reproduce this with 7.10.3 and 8-rc4. I've compiled a git repo with minimalistic example of this: https://github.com/dredozubov/broken-instances-minimal

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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