Commit 226d86d2 authored by Committed by Marge BotBrowse files
Do not add a 'solved dict' for quantified constraints
GHC has a wonderful-but-delicate mechanism for building recursive dictionaries by adding a goal to the "solved dictionaries" before solving the sub-goals. See Note [Solved dictionaries] in TcSMonad Ticket #17267 showed that if you use this mechanism for local /quantified/ constraints you can get a loop -- or even unsafe coerce. This patch fixes the bug. Specifically * Make TcSMonad.addSolvedDict be conditional on using a /top level/ instance, not a quantified one. * Moreover, we /also/ don't want to add a solved dict for equalities (a~b). * Add lots more comments to Note [Solved dictionaries] to explain the above cryptic stuff. * Extend InstanceWhat to identify those strange built-in equality instances. A couple of other things along the way * Delete the unused Type.isIPPred_maybe. * Stop making addSolvedDict conditional on not being an impolicit parameter. This comes from way back. But it's irrelevant now because IP dicts are never solved via an instance.