fyi, below you can find an expanded list of related links.
I do appreciate the new sandboxing feature in 1.18 and I think it is a great feature, but it is not addressing the core issue here: adding support for multiple instances of the same version. It also seems to me that supporting this would be a relatively low hanging fruit that could have a big impact to the whole ecosystem once addressed. So why not address it? I don't have enough exposure to Ghc or Cabal to make a difference here, now (what is needed is an elaboration of the issues together with a minimal solution), but I would be willing to participate in a supporting role (testing, reviewing, and minor bug fixes).
GSoC 2012 - Enable GHC to use multiple instances of a package for compilation - Philipp Schuster
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