[GHC] #12919: Equality not used for substitution

[GHC]

#12919: Equality not used for substitution
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        Reporter:  int-index         |                Owner:  goldfire
            Type:  bug               |               Status:  closed
        Priority:  highest           |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  fixed             |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  typecheck/should_compile/T12919
      Blocked By:                    |             Blocking:  14441
 Related Tickets:  #13643            |  Differential Rev(s):  Phab:D3848
       Wiki Page:                    |
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Comment (by Marge Bot <ben+marge-bot@…>):

 In [changeset:"2b90356d26b4699227816ad9424e766eccdb6c36/ghc"
 2b90356d/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2b90356d26b4699227816ad9424e766eccdb6c36"
 Fix #14729 by making the normaliser homogeneous

 This ports the fix to #12919 to the normaliser. (#12919 was about
 the flattener.) Because the fix is involved, this is done by
 moving the critical piece of code to Coercion, and then calling
 this from both the flattener and the normaliser.

 The key bit is: simplifying type families in a type is always
 a *homogeneous* operation. See #12919 for a discussion of why
 this is the Right Way to simplify type families.

 Also fixes #15549.

 test case: dependent/should_compile/T14729{,kind}
            typecheck/should_compile/T15549[ab]
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12919#comment:26>
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