[GHC] #13537: Adding underscore (m :: _) to pattern variable makes it fail

[GHC]

#13537: Adding underscore (m :: _) to pattern variable makes it fail
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      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 simonpj):

 Consider this:
 {{{
 data T where
   MkT :: (forall a. a->a) -> T

 f (MkT (g :: Int->Int)) = ..
 }}}
 Is this ok?  Well yes.  The argument of `MkT` has type `forall a. a->a`,
 but it can be instantiated to the type `Int->Int`, so we can legitimately
 end up with `g :: Int->Int`.  In FC it'd look like
 {{{
 f x = case x of MkT (g2 :: forall a.a->a) ->
       let g::Int->Int  =  g2 @ Int
       in ...
 }}}
 Now you want to force the type of the pattern to be `_`.  But since GHC
 only supports predicative polymorphism, `_` must be a monotype.  So GHC
 instantiates `forall a. a->a` to `alpha -> alpha` (for some new
 unification variable alpha), and proceeds.

 And that is what you see in the first message "Found type wildcard...".
 But alas of course, in your example we need `m` to be a polytype, and
 making it a monotype leads to a subsequent rather obscure error message.

 Does that help?

 Is there some way we could improve the user manual?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13537#comment:2>
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