[GHC] #15361: Error message displays redundant equality constraint

[GHC]

#15361: Error message displays redundant equality constraint
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  8.4.3
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14394            |  Differential Rev(s):  Phab:D5002
       Wiki Page:                    |
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Comment (by Krzysztof Gogolewski <krz.gogolewski@…>):

 In [changeset:"c552feea127d8ed8cbf4994a157c4bbe254b96c3/ghc"
 c552feea/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="c552feea127d8ed8cbf4994a157c4bbe254b96c3"
 Suppress redundant givens during error reporting

 Summary:
 When GHC reports that it cannot solve a constraint in error
 messages, it often reports what given constraints it has in scope.
 Unfortunately, sometimes redundant constraints (like `* ~ *`, from
 #15361) can sneak in. The fix is simple: blast away these redundant
 constraints using `mkMinimalBySCs`.

 Test Plan: make test TEST=T15361

 Reviewers: simonpj, bgamari

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #15361

 Differential Revision: https://phabricator.haskell.org/D5002
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15361#comment:10>
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