Cabal submodule bump is needed
Cabal HEAD has recently been somewhat broken (https://github.com/haskell/cabal/issues/5318) and the current submodule is pointing to a buggy commit. This only manifests when installing
build-type: Custom packages, because that uses the bad code from the bootstrap Cabal;
build-type: Simple will use good code from the external
Please bump the Cabal submodule to fix this. I've verified that the observed problem (
happy breaking) goes away when this is done.