[GHC] #15462: fixST for lazy ST is a bit wrong

[GHC]

#15462: fixST for lazy ST is a bit wrong
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Core           |           Version:  8.4.3
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect result
  Unknown/Multiple                   |  at runtime
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #15349
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 #15349 exposed a problem with repeated computations in invocations of
 `fixST` that should fail but can instead produce answers or even subvert
 safe Haskell. That has been fixed for strict `ST`, but not yet for lazy
 `ST`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15462>
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