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  • Michael Peyton Jones's avatar
    Visibility: handle multiple units with the same name · 8a20bfc2
    Michael Peyton Jones authored and Marge Bot's avatar Marge Bot committed
    Fixes #16228. The included test case is adapted from the reproduction in
    the issue, and fails without this patch.
    
    ------
    
    We compute an initial visilibity mapping for units based on what is
    present in the package databases. To seed this, we compute a set of all
    the package configs to add visibilities for.
    
    However, this set was keyed off the unit's *package name*. This is
    correct, since we compare packages across databases by version. However,
    we would only ever consider a single, most-preferable unit from the
    database in which it was found.
    
    The effect of this was that only one of the libraries in a Cabal package
    would be added to this initial set. This would cause attempts to use
    modules from the omitted libraries to fail, claiming that the package
    was hidden (even though `ghc-pkg` would correctly show it as visible).
    
    A solution is to do the selection of the most preferable packages
    separately, and then be sure to consider exposing all units in the
    same package in the same package db. We can do this by picking a
    most-preferable unit for each package name, and then considering
    exposing all units that are equi-preferable with that unit.
    
    ------
    
    Why wasn't this bug apparent to all people trying to use sub-libraries
    in Cabal? The answer is that Cabal explicitly passes `-package` and
    `-package-id` flags for all the packages it wants to use, rather than
    relying on the state of the package database. So this bug only really
    affects people who are trying to use package databases produced by Cabal
    outside of Cabal itself.
    
    One particular example of this is the way that the
    Nixpkgs Haskell infrastructure provides wrapped GHCs: typically these
    are equipped with a package database containing all the needed
    package dependencies, and the user is not expected to pass
    `-package` flags explicitly.
    8a20bfc2