[GHC] #13959: substTyVar's definition is highly dubious

[GHC]

#13959: substTyVar's definition is highly dubious
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3732
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D3732


Comment:

 Happily, it passes `./validate`! I also ran `./validate --slow`, and I
 couldn't observe any //additional// `ASSERT` failures from this change
 (there were some `ASSERT` failures related to the substitution invariant
 before and after this chance, but importantly, the number of them remain
 unchanged with this patch).

 Since I'm convinced this will just work, I've submitted Phab:D3732 to
 Phab.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13959#comment:6>
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