This was disabled basically by accident before.
The links are jacked, but that was is true for every package; it is not
unique to this PR. I fixed it upstream here:
https://github.com/haskell/haddock/pull/1482
but it's not in any release distributions yet I don't think.
Fixes#171841
Otherwise attempt to build ghcHEAD from local checkout fails as:
$ nix build -L --impure --expr 'with import ~/nm {}; haskell.compiler.ghcHEAD.overrideAttrs (oa: { src = ./.; patches = []; nativeBuildInputs = oa.nativeBuildInputs ++ [ git ]; })' --keep-failed
...
ghc> checking C++ standard library flavour... ./configure: line 11487: /nix/store/r7r10qvsqlnvbzjkjinvscjlahqbxifl-gcc-wrapper-11.3.0/bin/cxx: No such file or directory
I think 'cxx' is not provided by stdenv.