I can't say I know the full details of the interactions, but the Cabal library is essentially the only bootlib that not only can safely (I don't think I've ever heard of anyone getting into trouble) be upgraded, but often is *recommended* for upgrade to support newer cabal-install. I think this is because ghc only uses a small portion of it that has been stable for a long time (avoiding data incompatibility) and it's compiled in (so no link time strangeness). The only issues that come to mind would be if someone were to write TH that itself explicitly called out to Cabal, or maybe some complex ghc-as-a-library stuff in a program explicitly using Cabal itself.
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