#13766: Confusing "redundant pattern match" in 8.0, no warning at all in 8.2 -------------------------------------+------------------------------------- Reporter: edsko | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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 RyanGlScott): I talked with mpickering about this privately, but I'll recap the conversation here as well. This may be expected behavior the sense of comment:5, but I still find it unsettling. The code that adb565aa74582969bbcc3b411d6d518b1c76c3cf targeted was of the form: {{{#!hs f = case [] of (_:_) -> case () of a -> undefined }}} Here, it'll warn that the `(_:_)` match is redundant: {{{ Pattern match is redundant In a case alternative: (_ : _) -> ... }}} And thanks to the aforementioned commit, it will //not// warn about the inaccessible `a -> undefined` case. This is definitely a Good Thing. A similar scenario arises in the code in this example: {{{#!hs instance Int ~ Bool => C Int where c = id }}} Because `Int ~ Bool` is insoluble (and thus all the code inside the instance is inaccessible), it won't bother printing out a warning that `c` is inaccessible. But this is a tad skeevy. In the former example, there was a separate warning that hinted that you were doing something questionable. In the latter example, however, there's no warning at all to point out the dubious nature of your code! Therefore, I think a satisfactory conclusion to this bug would be to come up with a suitable warning about the `Int ~ Bool` constraint. Whether that's the purview of #12694 or some other ticket, I'm not sure. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13766#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler