[GHC] #10820: Provide a way to detect what extensions are enabled via TH

[GHC]

#10820: Provide a way to detect what extensions are enabled via TH
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        Reporter:  spinda            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Template Haskell  |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by spinda):

 Would it work to have template-haskell depend on ghc and pull the type
 from DynFlags? My guess: yes.

 Wouldn't this create a mutually recursive package dependency, as {{{ghc}}}
 already depends on {{{template-haskell}}}? (see
 [https://github.com/ghc/ghc/blob/b4ed13000cf0cbbb5916727dad018d91c10f1fd8/compiler/ghc.cabal.in#L55
 ghc.cabal.in] and
 [https://github.com/ghc/ghc/blob/master/compiler/typecheck/TcSplice.hs#L105
 TcSplice])

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10820#comment:3>
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