[GHC] #15231: UndecidableInstances validity checking is wrong in the presence of QuantifiedConstraints

[GHC]

#15231: UndecidableInstances validity checking is wrong in the presence of
QuantifiedConstraints
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4819
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"91822e4eee295a42f69489c7e9e878b296e897bc/ghc"
 91822e4e/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="91822e4eee295a42f69489c7e9e878b296e897bc"
 Add "quantified constraint" context in error message, fix #15231.

 This patch adds "quantified constraint" context in error message when
 UndecidableInstances checking fails for quantified constraints.
 See Trac #15231:comment#1.

 This patch also pretty-prints the instance head for better error messages.

 Test Plan: make test TEST="T15231"

 Reviewers: bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, rwbarton, thomie, carter

 GHC Trac Issues: #15231

 Differential Revision: https://phabricator.haskell.org/D4819
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15231#comment:4>
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