[GHC] #12387: Template Haskell ignores class instance definitions with methods that don't belong to the class

[GHC]

#12387: Template Haskell ignores class instance definitions with methods that don't
belong to the class
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Well, perhaps the phrase "postponing" isn't quite accurate. It turns out
 that for associated type family instances, membership in a class is tested
 //twice//, once during renaming and once during typechecking. comment:3
 shows an example of the latter, and this demonstrates the former:

 {{{#!hs
 {-# LANGUAGE TypeFamilies #-}
 module Foo where

 import GHC.Generics

 instance Eq () where
   type Rep () = Maybe
 }}}
 {{{
 $ /opt/ghc/8.4.2/bin/ghc Bug.hs
 [1 of 1] Compiling Foo              ( Bug.hs, Bug.o )

 Bug.hs:7:8: error:
     ‘Rep’ is not a (visible) associated type of class ‘Eq’
   |
 7 |   type Rep () = Maybe
   |        ^^^
 }}}

 So it might suffice to just check class membership for class methods
 additionally during typechecking, just like we do for associated type
 family instances.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12387#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler