[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:  8.2.1
       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:  #12918            |  Differential Rev(s):  Phab:D2682,
       Wiki Page:                    |  Phab:D2786
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"eec02ab7c8433465cc8d6be0a8889e7c6a222fb0/ghc"
 eec02ab7/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="eec02ab7c8433465cc8d6be0a8889e7c6a222fb0"
 Give concrete example for #12784 in 8.0.2 release notes

 Summary:
 We mentioned that there were "some programs" that failed to typecheck
 due to #12784, but given how surprisingly common this issue has been, it'd
 be prudent to at least give one example of the bug in the release notes.

 Reviewers: simonpj, bgamari, austin, rwbarton

 Reviewed By: rwbarton

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2786

 GHC Trac Issues: #12784
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12784#comment:27>
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