[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):

 That being said, I'm sympathetic to the desire to have a consistent,
 predictable way to determine when exactly a `forall`'d variable from a
 type signature is brought into scope, so I don't want to hammer too hard
 on this point unless goldfire is on board too.

 I'm sure goldfire has a convincing example in comment:8, but I'm having
 trouble understanding what makes it convincing. I'm grasping at straws
 here, but is the concern that something like this (with invisible patterns
 written out explicitly) should "intuitively" typecheck:

 {{{#!hs
 f1 :: forall a. a -> Int -> forall b. (a, b)
 f1 @a x i @b = const (x, undefined :: b) i
 }}}

 But in the eta-reduced version:

 {{{#!hs
 f2 :: forall a. a -> Int -> forall b. (a, b)
 f2 @a x = const (x, undefined :: b {-?-})
 }}}

 `b` isn't bound by an invisible pattern? Or is there some other reason why
 this should be considered "strange"?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14288#comment:13>
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