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    Filter instance visibility based on set of visible orphans, fixes #2182. · 4c834fdd
    Edward Z. Yang authored
    
    
    Summary:
    Amazingly, the fix for this very old bug is quite simple: when type-checking,
    maintain a set of "visible orphan modules" based on the orphans list of
    modules which we explicitly imported.  When we import an instance and it
    is an orphan, we check if it is in the visible modules set, and if not,
    ignore it.  A little bit of refactoring for when orphan-hood is calculated
    happens so that we always know if an instance is an orphan or not.
    
    For GHCi, we preinitialize the visible modules set based on the list of
    interactive imports which are active.
    
    Future work: Cache the visible orphan modules set for GHCi, rather than
    recomputing it every type-checking round.  (But it's tricky what to do when you
    /remove/ a module: you need a data structure a little more complicated than
    just a set of modules.)
    
    Signed-off-by: default avatarEdward Z. Yang <ezyang@cs.stanford.edu>
    
    Test Plan: new tests and validate
    
    Reviewers: simonpj, austin
    
    Subscribers: thomie, carter
    
    Differential Revision: https://phabricator.haskell.org/D488
    
    GHC Trac Issues: #2182
    4c834fdd