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

[GHC]

#10547: feature request: expanding type synonyms in error messages
-------------------------------------+-------------------------------------
        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:                    |  Phab:D2198
-------------------------------------+-------------------------------------

Comment (by osa1):

 OK, this should be fixed now. I'm currently validating the patch locally.

 As for why not enable this by default: The algorithm is looking through
 all expansions, so if the first type can be expanded `n` and second type
 `m` times, it takes `O(n * m)` steps to find a version of the error
 message that has minimum amount of expansions. When fixing this bug I
 realized that type synonym expansion is probably fast enough, and in
 practice `n` and `m` will not be too big, so maybe it's OK to enable this
 by default. We should probably just generate some pathological cases and
 look at compile times with and without `-fprint-expanded-synonyms`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10547#comment:17>
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