[GHC] #12972: Missed specialisation opportunity with phantom type class parameter?

[GHC]

#12972: Missed specialisation opportunity with phantom type class parameter?
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           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I am unsure of my analysis of this code fragment. It seems like we could
 do a better job optimising `test3`. First the code, then the analysis at
 the bottom.

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE RoleAnnotations #-}
 {-# LANGUAGE IncoherentInstances #-}
 module Foo where

 data Proxy a = Proxy

 --type role Phantom phantom nominal

 class Phantom x a | a -> x where
   method :: a
   method1 :: a

 instance Phantom x (Proxy x) where
   method = Proxy
   method1 = Proxy

 -- This doesn't optimise

 test3 :: Phantom x (Proxy x) => Proxy x
 test3 = method

 -- This does optimise

 instance Phantom Char Int where
   method = 5
   method1 = 5

 test4 :: Phantom x Int => Int
 test4 = method
 }}}

 Here is the relevant part of the core

 {{{#!hs
 -- RHS size: {terms: 4, types: 9, coercions: 0}
 test3
 test3 = \ @ x_ayL $dPhantom_ayS -> method $dPhantom_ayS

 -- RHS size: {terms: 3, types: 5, coercions: 0}
 test4
 test4 = \ @ x_ayz _ -> $cmethod1_az4
 }}}

 In `test4` the dictionary selector `method` is eliminated but in the
 analogous case `test3` where `x` is used in both arguments then `method`
 is not specialised. It seems that we could do a similar specialisation and
 ultimately replace the dictionary with `Proxy` as `x` is phantom.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12972>
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