[GHC] #10278: ApiAnnotations : Nested forall loses forall annotation

[GHC]

#10278: ApiAnnotations : Nested forall loses forall annotation
-------------------------------------+-------------------------------------
              Reporter:  alanz       |             Owner:  alanz
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:  7.10.2
             Component:  Compiler    |           Version:  7.10.1
  (Parser)                           |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
  ApiAnnotations                     |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 When parsing

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}

 extremumNewton :: forall tag. forall tag1.
                    tag -> tag1 -> Int
 extremumNewton = undefined
 }}}

 The parser attaches an `AnnForall` to the second `forall`, which appears
 as a nested `HsForAllTy`.

 Somewhere this nesting is flattened, and the tyvarbndrs are collapsed into
 a single `HsForAllTy`. In this process the second `AnnForAll` loses its
 anchor in the AST.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10278>
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