[GHC] #15558: Locally handling an empty constraint

[GHC]

#15558: Locally handling an empty constraint
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        Reporter:  Ericson2314       |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  ⊥
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I'm not quite sure I understand what this ticket is about, but it's worth
 noting that inaccessible code turned from an error to a warning in GHC
 8.6.1, so your code does in fact compile as-is on 8.6.1:

 {{{
 $ /opt/ghc/8.6.1/bin/ghci Bug.hs
 GHCi, version 8.6.0.20180810: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Asdf             ( Bug.hs, interpreted )

 Bug.hs:49:3: warning: [-Winaccessible-code]
     • Couldn't match type ‘[b]’ with ‘Int’
       Inaccessible code in
         a pattern with constructor: X :: Sing Int, in a case alternative
     • In the pattern: X
       In a case alternative: X -> Bar 0 undefined
       In the expression:
         \case
           X -> Bar 0 undefined
           Y -> Bar [] False
    |
 49 |   X -> Bar 0 undefined -- can't make this happy
    |   ^
 }}}

 Does this satisfy your use case?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15558#comment:5>
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