[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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Comment (by Ben Gamari <ben@…>):

 In [changeset:"0abe7361249b0b4dc43dc66547451da8916b30bf/ghc"
 0abe7361/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0abe7361249b0b4dc43dc66547451da8916b30bf"
 Don't replace type family instances with the same LHS in GHCi (#7102)

 This fixes the easy part of #7102 by removing the logic that lets the
 user replace a type family instance with a new one with the same LHS.
 As discussed on that ticket, this is unsound in general. Better to have
 the user redefine the type family from scratch.

 The example from comment:7 involving loading modules into ghci is not
 fixed yet; it actually doesn't rely on the instances having the same LHS.
 This commit adds an expect_broken test for that example as well.

 Test Plan: T7102a for the fix; T7102 is the test not fixed yet

 Reviewers: dfeuer, austin, bgamari, goldfire

 Reviewed By: dfeuer

 Subscribers: dfeuer, thomie

 Differential Revision: https://phabricator.haskell.org/D2994
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7102#comment:14>
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