[GHC] #15629: "No skolem info" panic (GHC 8.6 only)

[GHC]

#15629: "No skolem info" panic (GHC 8.6 only)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.6.1-beta1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Found it. It's commit 042df603cbb5a77ec13ccfec2ce7bad2bb940aae:

 {{{
 From 042df603cbb5a77ec13ccfec2ce7bad2bb940aae Mon Sep 17 00:00:00 2001
 From: Richard Eisenberg <rae at cs.brynmawr.edu>
 Date: Thu, 5 Jul 2018 14:21:43 -0400
 Subject: [PATCH] Unwrap casts before checking vars in eager unifier

 Previously, checking whether (tv |> co) ~ (tv |> co) got deferred,
 because we looked for vars before stripping casts. (The left type
 would get stripped, and then tv ~ (tv |> co) would scare the occurs-
 checker.)

 This opportunity for improvement presented itself in other work.
 This is just an optimization. Some programs can now report more
 errors simultaneously.
 }}}

 Pretty impressive for "just an optimization" :)

 And yes, we should absolutely add a regression test for this, since I
 couldn't find another program of this caliber in the test suite currently.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15629#comment:3>
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