[GHC] #13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not used.

[GHC]

#13649: RebindableSyntax causes type errors when 'fail' is not defined, even if not
used.
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        Reporter:  AaronFriel        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.2.1-rc1
      Resolution:                    |             Keywords:
                                     |  RebindableSyntax
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"2fcb5c5c3f6c5a5936eeb5dc07b476e5737f12ad/ghc"
 2fcb5c5c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2fcb5c5c3f6c5a5936eeb5dc07b476e5737f12ad"
 compiler: Do not look up fail in RnExpr if bind pattern is irrefutible.

 Adds a check in `rnStmt`, in sub-expr `getFailFunction`, to determine if
 the pattern of a bind statement is irrefutible. If so, skip looking up
 the `fail` name.

 Reviewers: austin, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #13649

 Differential Revision: https://phabricator.haskell.org/D3553
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13649#comment:6>
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