[GHC] #12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures

[GHC]

#12784: Typechecker regression in GHC 8.0.2 involving DefaultSignatures
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
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:D2682
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"2e8463b232054b788b73e6551947a9434aa76009/ghc"
 2e8463b2/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2e8463b232054b788b73e6551947a9434aa76009"
 Update 8.0.2 release notes for #12784

 Summary:
 The fix for #12220 exposed some ill-typed programs which passed the
 typechecker
 in GHC 8.0.1 but now fail to typecheck in GHC 8.0.2. It's a bit difficult
 to
 characterize what exactly triggers this bug, but we at least have a
 minimal
 example and a simple fix to illustrate the problem and solution, so let's
 add that the the 8.0.2 release notes to advertise this change.

 Resolves #12784.

 Reviewers: rwbarton, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2682

 GHC Trac Issues: #12784
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12784#comment:8>
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