[GHC] #11121: Introduce `-XTemplateHaskellQuotes` pragma for subset of TH

[GHC]

#11121: Introduce `-XTemplateHaskellQuotes` pragma for subset of TH
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        Reporter:  hvr               |                Owner:
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.0.1
       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:  #10382, #11102    |  Differential Rev(s):  Phab:D1511
       Wiki Page:                    |
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Changes (by hvr):

 * status:  new => patch


Old description:

> TODO: describe why we need this

New description:

 Cabal needs to be able to detect whether a full or partial TemplateHaskell
 implementation is available. However, if `-XTemplateHaskell` has no clear
 meaning we can't properly specify which language extensions a package
 requires nor make accurate decisions based on the availability of compiler
 features.

 Moreover, the quotation subset of TemplateHaskell seems to be SafeHaskell
 compatible, so it makes sense to split it off into a sub-extension.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11121#comment:3>
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