[GHC] #11156: Type-changing record update catch-all in sum type doesn't typecheck

[GHC]

#11156: Type-changing record update catch-all in sum type doesn't typecheck
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        Reporter:  afarmer           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:  invalid           |             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 afarmer):

 Yeah, I figured it wouldn't be easy to fix, and the workaround isn't too
 onerous.

 However, it does seem like a bit of a wart. I had to stare at it for many
 minutes before I realized what was probably going on... the error seems
 really non-obvious unless you understand how record updates desugar, so I
 just wanted to document it.

 I wondered if the new overlapping/missing patterns algorithm could be
 adapted to spot the impossible case, but I'm sure the effort-to-payoff
 ratio is much too high.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11156#comment:3>
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