[GHC] #13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2

[GHC]

#13708: Panic! (the "impossible" happened) bug in GHC 8.2.1 rc2
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        Reporter:  deech             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"d6461f9684f6f758320a5e5afbf0634fcc2996a5/ghc"
 d6461f96/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d6461f9684f6f758320a5e5afbf0634fcc2996a5"
 Handle type-lets better

 Core allows non-recursive type-lets, thus

    let a = TYPE ty in ...

 They are substituted away very quickly, but it's convenient for
 some passes to produce them (rather than to have to substitute
 immediately).

 Trac #13708 tried the effect of not running the simplifer at all
 (a rather bizarre thing to do, but still).  That showed that some
 passes crashed because they always treated a let-bounder binder
 as an Id.  This patch adds some easy fixes.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13708#comment:10>
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