Commits on Mar 16, 2019

1. generic Haskell builder: don't copy packages from GHC …

   In order to build the package databases that we will use when compiling
   a Haskell package, we iterate over the relevant dependencies, and if
   they contain a package db, we copy its contents over.
   
   So far so good, except when one of those dependencies is GHC. This
   doesn't happen ordinarily, but it will happen when we construct the
   package database for compiling `Setup.hs`.  This is compiled for the
   build architecture, so we get the build deps, including both the native
   and the cross GHC (if there is one).
   
   In this case, we end up copying the packages from the GHC's package
   database. This is at best unnecessary, since we will get those packages
   from the GHC when we compile with it.
   
   At worst, however, this is semantically questionable. We can end up
   having multiple copies of e.g. Cabal with the same version, but
   (potentially) different contents. At the moment, GHC will expose one of
   these at semi-random depending on which one it looks at "first".
   However, there is a MR open [in
   GHC](https://gitlab.haskell.org/ghc/ghc/merge_requests/545) which as a
   side effect will instead expose both, leading to ambiguous module
   warnings (which is not unreasonable, since it *is* ambiguous).
   
   So what can we do about it? The simplest solution is just to not copy
   the package databases from GHC. GHC is special in this regard, so I
   think it's okay to treat it specially.
   
   This PR should have no effect on anything now, but will prevent any
   breakage when/if the GHC patch lands.
   
   Closes #57706.

   

   michaelpj authored and peti committed Mar 16, 2019
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