[GHC] #15772: Strange constraint error that disappears when adding another top-level declaration

[GHC]

#15772: Strange constraint error that disappears when adding another top-level
declaration
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        Reporter:  kosmikus          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.6.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 This looks subtle.  I think that `Note [FunEq occurs-check principle]` may
 be implicated, and I'm worried that there's something subtle about solve
 order.

 FWIW, `-fdefer-type-errors` forces implications to be built more
 aggressively, and that too makes it work.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15772#comment:4>
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