[GHC] #7102: Type family instance overlap accepted in ghci

[GHC]

#7102: Type family instance overlap accepted in ghci
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        Reporter:  exbb2             |                Owner:  rwbarton
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  GHCi              |              Version:  7.4.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:  T7102a, T7102
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2994
       Wiki Page:                    |
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Changes (by rwbarton):

 * testcase:   => T7102a, T7102
 * differential:   => Phab:D2994


Comment:

 There are actually two issues in this ticket.

 * The original issue, that you can replace type family instances in ghci
 as in comment:1. There was special logic to allow this (copied from the
 case of class instances), so fixing it was just deleting code (that I was
 going to have to change anyways, for another issue). I added test `T7102a`
 for this one.

 * The issue in comment:7 is different, and doesn't rely on the two
 instances having the same LHS. It's apparently not that ghci is replacing
 one instance with the other one, but rather that it never checks that the
 two instances are consistent with each other. So we need an additional
 consistency check somewhere. I haven't fixed this yet, but I added a
 expect_broken test `T7102` for it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7102#comment:12>
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