It goes in `Cabal-syntax/src/Language/Haskell/Extensions.hs`, in `data
KnownExtension`. The parse table is built from the constructor names, and
`No` prefixes are handled automatically. Note that you'll need to also bump
the `cabal-version` (file format version) and add an entry to the file
format documentation (`doc/file-format-changelog.rst`).

On Sun, Mar 15, 2026 at 1:18 PM Simon Peyton Jones via ghc-devs <
[email protected]> wrote:

> Friends
>
> As part of the reinstallable base project I'm adding an
> extension "ImplicitKnownKeyNames" to GHC.  But if I put
> "NoImplicitKnownKeyNames" in base.cabal.in I get the error
>
> Warning: [unknown-extension] Unknown extensions: NoImplicitKnownKeyNames
>
> when building the base library.  I think this comes from cabal.
>
> How can I tell cabal about the new extension?
>
> Thanks
>
> Simon
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-- 
brandon s allbery kf8nh
[email protected]
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