[GHC] #13092: family instance consistency checks are too pessimistic

[GHC]

#13092: family instance consistency checks are too pessimistic
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        Reporter:  rwbarton          |                Owner:  rwbarton
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 I managed to break type family consistency in 8.0.1 using `ghc -c` alone,
 but apparently it was in some way that fooled the recompilation checker
 and it can't be reproduced from a clean build. So I don't know how I did
 it.

 In any case, it seems more logical for the type family instances in `A` to
 be checked against `B` while compiling `A`, rather than forcing every
 importer of `A` to do it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13092#comment:2>
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