Filter instance visibility based on set of visible orphans, fixes #2182.
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: Edward Z. Yang <email@example.com> Test Plan: new tests and validate Reviewers: simonpj, austin Subscribers: thomie, carter Differential Revision: https://phabricator.haskell.org/D488 GHC Trac Issues: #2182