[GHC] #13986: TypeApplications causes parse errors in @-patterns with certain Unicode characters

[GHC]

#13986: TypeApplications causes parse errors in @-patterns with certain Unicode
characters
-------------------------------------+-------------------------------------
        Reporter:  Tikhon            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
  (Parser)                           |             Keywords:
      Resolution:                    |  TypeApplications
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:                    |  https://phabricator.haskell.org/D4105
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"821adee12e89dbd0a52fde872b633e4e2e9051dc/ghc"
 821adee1/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="821adee12e89dbd0a52fde872b633e4e2e9051dc"
 Fix a bug in 'alexInputPrevChar'

 The lexer hacks around unicode by squishing any character into a 'Word8'
 and then storing the actual character in its state. This happens at
 'alexGetByte'.

 That is all and well, but we ought to be careful that the characters we
 retrieve via 'alexInputPrevChar' also fit this convention.

 In fact, #13986 exposes nicely what can go wrong: the regex in the left
 context of the type application rule uses the '$idchar' character set
 which relies on the unicode hack. However, a left context corresponds
 to a call to 'alexInputPrevChar', and we end up passing full blown
 unicode characters to '$idchar', despite it not being equipped to deal
 with these.

 Test Plan: Added a regression test case

 Reviewers: austin, bgamari

 Reviewed By: bgamari

 Subscribers: rwbarton, thomie

 GHC Trac Issues: #13986

 Differential Revision: https://phabricator.haskell.org/D4105
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13986#comment:4>
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