[GHC] #11102: `ghc --supported-extension` shall not list `TemplateHaskell` when unsupported

[GHC]

#11102: `ghc --supported-extension` shall not list `TemplateHaskell` when
unsupported
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              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 Rev(s):  phab:D1484
       Wiki Page:                    |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"98a4fa5fb740ce43a1787eb5730e35b463a2de79/ghc" 98a4fa5/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="98a4fa5fb740ce43a1787eb5730e35b463a2de79"
 DynFlags: Don't list TemplateHaskell as supported if it's not

 `-XTemplateHaskell` requires GHCi; don't show it in
 `--supported-extensions` if it's not supported.

 Together with https://github.com/haskell/cabal/issues/2644
 this allows to finally fix a long-standing annoyance in cabal
 of not being able to detect availability of TH in build-plans.

 Fixes #11102

 See also https://github.com/haskell/cabal/pull/2930 which provides
 emulation of this feature for older GHCs.

 Reviewed By: bgamari

 Differential Revision: https://phabricator.haskell.org/D1484
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11102#comment:2>
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