[GHC] #10547: feature request: expanding type synonyms in error messages

[GHC]

#10547: feature request: expanding type synonyms in error messages
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        Reporter:  osa1              |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
                                     |  typecheck/should_fail/ExpandSynsFail1,2,3,4
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1016
       Wiki Page:                    |
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Comment (by ezyang):

 Thanks for the nice example. osa1, can you tell why it is slow in this
 case? Would it be easy to fix?

 If testing if there is an interesting expansion is slow, then it won't be
 possible to mention `-fprint-expanded-synonyms` since we won't be able to
 tell if there is an expansion or not (we could mention it unconditionally
 but that feels like bad UX.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10547#comment:14>
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