[GHC] #10533: Assertion failure on FD3 test in TcErrors (was: Typechecker behaves differently on Phab and on Mac)

[GHC]

#10533: Assertion failure on FD3 test in TcErrors
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.11
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------
Changes (by goldfire):

 * os:  MacOS X => Unknown/Multiple
 * component:  Compiler => Compiler (Type checker)


Old description:

> On my Mac, commit 61b96a86c5342fb1c850361177d60fe855d948f6 causes test
> `typecheck/should_compile/FD3` to fail. But Phab succeeded in validating
> this commit, as seen [https://phabricator.haskell.org/B4297 here]. (I
> don't believe that commit has anything to do with the failure -- it's
> just the snapshot I looked at.)
>
> This leads to two questions:
>
> 1. Why does this test fail on a Mac?
>
> 2. Why does the typechecker behave differently, at all, on two different
> platforms?

New description:

 Test case `typecheck/should_compile/FD3` leads to an `ASSERT`ion error:

 {{{
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 7.11.20150605 for x86_64-apple-darwin):
         ASSERT failed!
   file compiler/typecheck/TcErrors.hs line 321
   [[D] _ :: a_amP[sk] ~ (String, a_amP[sk]) (CNonCanonical)]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 FD3.log (attached) shows the error happening.

 (When originally posting this ticket, I was very confused. The first
 several comments, through comment:6, are now bogus. Please ignore.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10533#comment:7>
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