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`-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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