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

[GHC]

#14288: ScopedTypeVariables with nested foralls broken since 8.0.2
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Richard and I both think we should stick with the current behaviour, and
 specify it in the user manual.   That is, with `-XScopedTypeVariables`,
 and a signature
 {{{
 f :: forall a b c. blah
 <definition of f>
 }}}
 then `a`, `b` and `c` scope over f's definition, but no other type
 variables do. This applies unconditionally, even if `blah` starts with
 another `forall`.

 The `forall` must appear literally, not hidden by a type synonym. Eg. no
 variables scope here
 {{{
 type Foo = forall a b. a -> b -> Int

 f :: Foo
 f = e -- a and b are not in scope here
 }}}

 This rule is simple, and does not restrict expressiveness.  Otherwise we
 land up trying to decide where to draw the line (and having to explain
 where it is drawn, in the user manual). Eg
 {{{
 f :: forall a. forall b. blah
 f :: forall a. a -> forall b. blah
 f :: forall a. Eq a => forall b. blah
 }}}
 Would someone care to update the user manual?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14288#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler