[GHC] #15637: Ambiguous type variables in GeneralisedNewtypeDeriving

[GHC]

#15637: Ambiguous type variables in GeneralisedNewtypeDeriving
-------------------------------------+-------------------------------------
           Reporter:  i-am-tom       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
  GeneralisedNewtypeDeriving,        |
  GeneralizedNewtypeDeriving         |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When deriving the `C` instance in the following code:

 {{{#!hs
 {-# LANGUAGE AllowAmbiguousTypes #-}
 {-# LANGUAGE GeneralisedNewtypeDeriving #-}
 module Test where

 class C a where f :: String

 instance C () where f = "foo"

 newtype T = T () deriving C
 }}}

 The following error occurs:

 {{{
 Test.hs:10:27: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘f’
       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 T -- Defined at Test.hs:10:27
         instance C () -- Defined at Test.hs:8:10
     • In the third argument of ‘GHC.Prim.coerce’, namely ‘f’
       In the expression: GHC.Prim.coerce @String @String f :: String
       In an equation for ‘f’:
           f = GHC.Prim.coerce @String @String f :: String
       When typechecking the code for ‘f’
         in a derived instance for ‘C T’:
         To see the code I am typechecking, use -ddump-deriv
    |
 10 | newtype T = T () deriving C
    |                           ^
 }}}

 ... and the following core is produced:

 {{{
 ==================== Derived instances ====================
 Derived class instances:
   instance Test.C Test.T where
     Test.f
       = GHC.Prim.coerce @GHC.Base.String @GHC.Base.String Test.f ::
           GHC.Base.String


 Derived type family instances:
 }}}

 The problem seems to be that the `a` should have been set to `()` within
 the coerced instance. I've been working round this with a `newtype X a = X
 String` as the result value so that the `a` is present in the signature,
 but I think this is a bug; should a more specialised instance be
 generated? I hope this is enough of an explanation!

 Thanks,
 Tom

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15637>
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