[GHC] #15549: Core Lint error with EmptyCase

[GHC]

#15549: Core Lint error with EmptyCase
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |             Keywords:  TypeFamilies,
      Resolution:                    |  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |  typecheck/should_compile/T15549[ab]
      Blocked By:                    |             Blocking:
 Related Tickets:  #14729            |  Differential Rev(s):
       Wiki Page:                    |  https://gitlab.haskell.org/ghc/ghc/merge_requests/208
-------------------------------------+-------------------------------------

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/15549#comment:11>
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