[GHC] #15899: Testcase tcfail158 is broken

[GHC]

#15899: Testcase tcfail158 is broken
-------------------------------------+-------------------------------------
        Reporter:  RolandSenn        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Test Suite        |              Version:  8.7
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12906            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * related:   => #12906


Comment:

 It's worth noting that `tcfail158` will in fact start reporting these
 errors once/if the fix for #12906 (Phab:D5338) lands.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15899#comment:1>
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