ok, I think I agree with your diagnosis
one of the problems with warning in the default package mode (without -hide-all-packages) is that, in most databases, there are lots of inconsistent versions of things, so it would be warning basically all the time
so you'd have to implement this after you handled imports; THEN check
#15425: GHC does not warn when two conflicting packages with same name are given via a dependent package and -package-db -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Build System | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nh2): `ezyang__` from IRC: that the transitive closures of packages make sense -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15425#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler