[GHC] #9378: Make unknown LANGUAGE pragmas a warning, not an error

[GHC]

#9378: Make unknown LANGUAGE pragmas a warning, not an error
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              Reporter:  goldfire    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by hvr):

 Well, then there's the other thing that turning such a pragma into warning
 is in conflict with the
 [https://www.haskell.org/onlinereport/haskell2010/haskellch12.html#x19-19100012.3
 Haskell 2010 Report] stating explicitly that:

 > If a Haskell implementation does not recognize or support a particular
 language feature that a source file requests (or cannot support the
 combination of language features requested), any attempt to compile or
 otherwise use that file with that Haskell implementation **must fail with
 an error**.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9378#comment:3>
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