[GHC] #14101: Type synonyms can make roles too conservative

[GHC]

#14101: Type synonyms can make roles too conservative
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  roles
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8234             |  Differential Rev(s):  Phab:D3838
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"0bb1e84034a12d7f700b48fca6710c01bd08f397/ghc"
 0bb1e840/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0bb1e84034a12d7f700b48fca6710c01bd08f397"
 Expand type synonyms during role inference

 Summary:
 During role inference, we need to expand type synonyms, since
 oversaturated applications of type synonym tycons would otherwise have
 overly
 conservative roles inferred for its arguments.

 Fixes #14101.

 Test Plan: ./validate

 Reviewers: goldfire, austin, bgamari

 Reviewed By: goldfire

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #14101

 Differential Revision: https://phabricator.haskell.org/D3838
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14101#comment:11>
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