[GHC] #15637: Ambiguous type variables in GeneralisedNewtypeDeriving

[GHC]

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

 Replying to [comment:1 simonpj]:
 > I suppose we could generate
 > {{{
 > Test.f = coerce @String @String (Test.f @())
 > }}}
 > that is, explicitly instantiating `f`.

 That would make this particular example work, yes.

 Note that this strategy would not support GND'ing //every// class under
 the sun that uses `AllowAmbiguousTypes`, such as the example from #14266,
 which uses an ambiguous type variable in a class context. But your idea
 would still be an improvement over the status quo, so I'm inclined to
 adopt it.

 > (Then maybe we could omit the second type arg to `coerce`; I'm not
 sure.)

 No, I don't think adding these extra type applications would change the
 fact that you need the second type argument to `coerce`. The reason for
 that is explained in `Note [GND and QuantifiedConstraints]`
 [http://git.haskell.org/ghc.git/blob/02edb8f2f973a8df26cfb6dfab0ef99a832f711f:/compiler/typecheck/TcGenDeriv.hs#l1823
 here]; the short of it is the we need the second type argument to `coerce`
 to support GND'ing classes like:

 {{{#!hs
 class C a where
   c :: Int -> forall b. b -> a
 }}}

 That fact doesn't change even in the presence of this ticket.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15637#comment:2>
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