#14813: EmptyCase thinks pattern match involving type family is not exhaustive, when it actually is -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: | PatternMatchWarnings Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): The line of code which is responsible for this infelicity is [http://git.haskell.org/ghc.git/blob/7c173b9043f7a9a5da46c5b0cc5fc3b38d1a7019... here], within `checkEmptyCase'`: {{{#!hs mb_candidates <- inhabitationCandidates fam_insts (idType var) }}} But this is checking the inhabitation candidates of the raw `idType` of `x`, which is `F b a` instead of `F 'False a`. It seems that what needs to happen is that we need to take the local dictionary evidence in scope (i.e., `liftD getDictsDs`)—in this example, `b ~ 'False`—and somehow use that to turn `F b a` into `F 'False a`. On the other hand, I have no idea how one would do this, as `liftD getDictsDs` gives you a `Bag EvVar` instead of, say, a substitution, it's far from clear to me how you'd "apply" a `Bag EvVar`... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14813#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler