[GHC] #12923: MultiParamTypeClasses + ExtendedDefaultRules

[GHC]

#12923: MultiParamTypeClasses + ExtendedDefaultRules
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        Reporter:  amindfv           |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2822
       Wiki Page:                    |
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Comment (by simonpj):

 On Phab you comment that this program is still rejected with ambiguous
 variables.
 {{{
 {-# LANGUAGE ExtendedDefaultRules #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE KindSignatures #-}
 module T12924 where

 import GHC.TypeLits

 data A (b :: [Symbol]) = A deriving Show

 class Works a (b :: [Symbol]) where
    works :: a -> A b

 instance Works Integer a where
    works _ = A

 addA :: A a -> A a -> A a
 addA A A = A

 test2 :: A x
 test2 = addA (works 5) (works 5)
 }}}
 The reason is described in `Note [ApproximateWC]` in `TcSimplify`, item
 (2) in that note.  It arose from Trac #8155.  We have a constraint
 {{{
 forall x. () => Num alpha, Works alpha x
 }}}
 but because we carefully make the `Works alpha x` prevent the `Num alpha`
 float out for defaulting, for reasons described in the Note.

 The rule is un-documented and indeed hard to explain. I'm quite inclined
 to back-pedal on the fix to #8155.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12923#comment:15>
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