I think you could mitigate this particular bad case if you did "path compression", that is instead of keeping the finst-flag, you remembered
#14759: ListSetOps WARNING causes tests to fail -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4628 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): the first "interesting" module - a module that either defines it's own family instances or imports two modules with different "interesting" modules Interesting idea. Do you feel inclined to work on it? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14759#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler