[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):

 Fyi, when you inform Cabal about the used language pragmas via `other-
 extensions` (which then still need to be declared on a per-module basis
 via pragmas), Cabal will refuse to build the project if your compiler
 doesn't provide all those extensions.  Moreover, there is tooling to
 generate the `other-extensions` list automatically from the `.hs` files
 making up a build-target.

 So I'm a bit sceptical changing the semantics of `LANGUAGE` to be a mere
 warning would interact badly with the Cabal tooling.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9378#comment:1>
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