[GHC] #12170: Add flag to control whether out of scope variables should be deferred with -fdefer-typed-holes

[GHC]

#12170: Add flag to control whether out of scope variables should be deferred with
-fdefer-typed-holes
-------------------------------------+-------------------------------------
           Reporter:  mpickering     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  newcomer       |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #10569
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 As a result of #10569, out of scope variables are turned into special type
 holes so that type information of
 the variable can be attached to the error message. However, this causes
 problems when used with `-fdefer-typed-holes` as scope errors have been
 turned into type errors and are now deferred when using this flag.

 Richard sums up the situation, "Even if someone is deferring type errors,
 I imagine most people don't want to defer scope errors". It would be good
 to add a flag which controls whether out of scope variables are also
 deferrable when used with existing deferral flags.

 Goal: Add a flag ` -fdefer-out-of-scope-variables` which controls whether
 out of scope variables are
 deferred when using `-fdefer-typed-holes`. It should be turned off by
 default.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12170>
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