[GHC] #14288: ScopedTypeVariables with nested foralls broken since 8.0.2

[GHC]

#14288: ScopedTypeVariables with nested foralls broken since 8.0.2
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        Reporter:  MikolajKonarski   |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  GHC rejects       |  (amd64)
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 As far as specifying how GHC should bring variables from nested `forall`s
 into scope, I don't think it's nearly as difficult as you're making it out
 to be. The algorithm would be as follows: when `ScopedTypeVariables` is
 enabled and an identifier with an explicit type signature `T` is being
 typechecked, then the set of variables `foralls(T)` is brought into scope,
 where:

 * `foralls(forall a_1 ... a_k. T) = {a_1, ..., a_k} ∪ foralls (T)`
 * `foralls(C => T) = foralls(T)`
 * `foralls(b -> c) = foralls(c)`

 Note that this doesn't expand type synonyms, so we needn't worry about
 your `Foo` example in comment:5 bringing variables into scope.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14288#comment:7>
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