[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 Ben Gamari <ben@…>):

 In [changeset:"85fcd035f73679927a0539d5f6c9b919517365e1/ghc"
 85fcd035/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="85fcd035f73679927a0539d5f6c9b919517365e1"
 Implement new -XTemplateHaskellQuotes pragma

 Since f16ddcee0c64a92ab911a7841a8cf64e3ac671fd / D876, `ghc-stage1`
 supports a subset of `-XTemplateHaskell`, but since we need Cabal to be
 able detect (so `.cabal` files can be specified accordingly, see also
 GHC #11102 which omits `TemplateHaskell` from `--supported-extensions`)
 whether GHC provides full or only partial `-XTemplateHaskell` support,
 the proper way to accomplish this is to split off the
 quotation/non-splicing `TemplateHaskell` feature-subset into a new
 language pragma `TemplateHaskellQuotes`.

 Moreover, `-XTemplateHaskellQuotes` is considered safe under SafeHaskell

 This addresses #11121

 Reviewers: goldfire, ezyang, dterei, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D1511

 GHC Trac Issues: #11121
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11102#comment:3>
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