[GHC] #10283: Make it possible to suppress warnings produced by -fdefer-type-errors

[GHC]

#10283: Make it possible to suppress warnings produced by -fdefer-type-errors
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              Reporter:  adamgundry  |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.10.1
             Component:  Compiler    |  Operating System:  Unknown/Multiple
  (Type checker)                     |   Type of failure:  None/Unknown
              Keywords:              |        Blocked By:
          Architecture:              |   Related Tickets:
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 At the moment, there seems to be no way to suppress warnings generated by
 `-fdefer-type-errors` (even `-w` doesn't work). It would be nice to be
 able to silence them, preferably via a new flag `-fno-warn-deferred-type-
 errors`.

 The motivation for this comes from using `-fdefer-type-errors` to write
 tests that certain definitions do not typecheck (which is very useful, and
 is a cheaper alternative to writing code to invoke GHC on a source file
 and analyse its output). In this situation, it is annoying to see a large
 number of warnings when compiling the testsuite.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10283>
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