[GHC] #10569: Optionally treat an out-of-scope variable like a typed hole

[GHC]

#10569: Optionally treat an out-of-scope variable like a typed hole
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        Reporter:  simonpj           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ertes):

 Simon, but `-fdefer-typed-holes` is a very useful development tool, and
 when I enable that flag, I'm expecting underscored identifiers to behave
 differently.

 Let me just say that ''for me'' this new feature is more of a stumbling
 block than a useful tool, and there are two ways to fix it: to make it
 optional, or to implement #12157 (warning priorities). I'm fine with these
 errors being warnings, as long as I can at least make them show up at the
 top. "Not in scope" errors are genuine errors in my workflow.

 Don't get me wrong: I'm in favour of turning as many errors as possible
 into warnings, but since we don't have a system to prioritise warnings
 right now, some things should better stay errors.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10569#comment:9>
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