[GHC] #15766: GeneralizedNewtypeDeriving should support classes with ambiguous types

[GHC]

#15766: GeneralizedNewtypeDeriving should support classes with ambiguous types
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           Reporter:  reinerp        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           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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 The following module has a class with an ambiguous type:

 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Ambig where

 class C a where
   c :: Int

 data A = A

 instance C A where c = 5

 newtype B = B A deriving C
 }}}

 Thanks to TypeApplications, this typeclass is still usable. However,
 GeneralizedNewtypeDeriving fails on it:

 {{{
 Ambig.hs:12:26: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘c’
       prevents the constraint ‘(C a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance C A -- Defined at Ambig.hs:10:10
         instance C B -- Defined at Ambig.hs:12:26
     • In the third argument of ‘GHC.Prim.coerce’, namely ‘c’
       In the expression: GHC.Prim.coerce @(Int) @(Int) c
       In an equation for ‘c’: c = GHC.Prim.coerce @(Int) @(Int) c
       When typechecking the code for ‘c’
         in a derived instance for ‘C B’:
         To see the code I am typechecking, use -ddump-deriv
    |
 12 | newtype B = B A deriving C
    |
 }}}

 It shouldn't.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15766>
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