[GHC] #16146: Trivial partial type signature kills type inference in the presence of GADTs

[GHC]

#16146: Trivial partial type signature kills type inference in the presence of
GADTs
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        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
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 goldfire):

 Another interesting example has cropped up. Re-consider

 {{{#!hs
 wurble :: _
 wurble x = x
 }}}

 This is accepted, because the type `_` allows for implicit generalization.

 But what about

 {{{#!hs
 wurble2 :: forall. _
 wurble2 x = x
 }}}

 To my surprise, this is accepted too, with `wurble2 :: forall a. a -> a`.
 Yet, I would have expected my `forall.` to say that there are no
 quantified variables in the type of `wurble2`.

 So, maybe another design is this: `foo :: _` can be generalized. (This is
 at odds with the fact that `foo` cannot have constraints.) But if there is
 an explicit `forall` in type of `foo`, we appeal to the forall-or-nothing
 rule and say that the variables quantified in the `forall` are precisely
 the quantified variables; no more can be inferred. Then, we can suppress
 quantification in `forall. _`. One advantage here is that this new design
 is backward compatible (though I'm not terribly worried about that here).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16146#comment:8>
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