[GHC] #15603: ref6 example from StaticPointers documentation doesn't type check

[GHC]

#15603: ref6 example from StaticPointers documentation doesn't type check
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The
 [https://downloads.haskell.org/~ghc/8.4.3/docs/html/users_guide/glasgow_exts.html?highlight=static%20pointers
 #extension-StaticPointers documentation] for `StaticPointers` contains the
 following example.

 {{{
 ref6 y = let x = 1 in static x
 }}}

 but this doesn't get accepted by the type checker.

 {{{
 sp.hs:27:23: error:
     • ‘x’ is used in a static form but it is not closed because it
       has a non-closed type because it contains the
       type variables: ‘p_a7KP’
     • In the expression: static x
       In the expression: let x = 1 in static x
       In an equation for ‘ref6’: ref6 y = let x = 1 in static x
    |
 27 | ref6 y = let x = 1 in static x
    |                       ^^^^^^^
 }}}

 I tested on 8.6.1, 8.4.3, 8.2.2, 8.02, 7.10.3 and all fail.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15603>
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