Support template-haskell-2.21.0.0

[locallycompact]

Caught by https://gitlab.horizon-haskell.net/package-sets/horizon-core/-/jobs/449605

[RyanGlScott]

The log you linked indicates that you are using GHC 9.9. Note that at present, both GHC 9.8 (which is soon to be released) and 9.9 bundle a copy of template-haskell that is called template-haskell-2.21.0.0, but these are not actually the same version of the library. The GHC 9.9 version of template-haskell introduced a breaking API change to the type of Code in template-haskell in this commit, which has not yet been reflected with a major version bump in the template-haskell library. (I'd expect the GHC maintainers to bump the template-haskell version to 2.22.0.0 shortly before GHC 9.10 is released.)

This means that guarding against this change with MIN_VERSION_template_haskell(2,21,0) is not correct, as that will then cause the library to fail to build with GHC 9.8. For this reason, the head.hackage patch for th-compat guards against this change using #if __GLASGOW_HASKELL__ >= 909 instead. This is a bit hacky, of course, but for the reasons mentioned above, we can't yet use something like MIN_VERSION_template_haskell(2,22,0).

I'd prefer to wait until template-haskell-2.22.0.0 is available before adding CPP to accommodate this. In the meantime, head.hackage offers a patched version of th-compat.