Template Haskell ignores class instance definitions with methods that don't belong to the class

{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Main where

import Language.Haskell.TH.Lib

data Foo = Foo

$(do d <- instanceD (cxt []) (conT ''Eq `appT` conT ''Foo)
            [funD 'compare [clause [] (normalB $ varE 'undefined) []]]
     return [d])

main :: IO ()
main = print $ Foo == Foo

$ /opt/ghc/8.0.1/bin/runghc Bug.hs 
Bug.hs:(9,3)-(11,15): Splicing declarations
    do { d_a2hL <- instanceD
                     (cxt [])
                     (conT ''Eq `appT` conT ''Foo)
                     [funD 'compare [clause [] (normalB $ varE 'undefined) []]];
         return [d_a2hL] }
  ======>
    instance Eq Foo where
      compare = undefined

Bug.hs:9:3: warning: [-Wmissing-methods]
    • No explicit implementation for
        either ‘==’ or ‘/=’
    • In the instance declaration for ‘Eq Foo’
Bug.hs: stack overflow

compare obviously doesn't belong to Eq, yet GHC happily accepts an Eq Foo instance with a definition for compare! Worse yet, there's now neither a definition for (==) nor (/=), so the default definition of (==) triggers an infinite loop, blowing the stack at runtime.

I don't know how pervasive this bug is. That is, I'm not sure if you could also attach associated type family instances, pattern synonyms, etc. that don't belong to the class either.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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