#15305: Erroneous "non-exhaustive pattern match" using nested GADT with strictness annotation -------------------------------------+------------------------------------- Reporter: jkoppel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Research Component: Compiler (Type | needed checker) | Version: 8.4.3 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): I think this would work fine for simple, non-recursive examples like the original program, but I fail to see how this would work for recursive examples like the program in comment:7. If you have the constraint `NonVoid(Abyss)`, how do you conclude that that is contradictory? Presumably, you'd need some sort of way to determine if there are any terminating inhabitants of `Abyss`. Unlike the `InL` example, where the type of its field alone (`Foo B`) is enough to conclude that it is uninhabitable, nothing about the type of `MkAbyss`'s field (`Abyss`) suggests that it is uninhabitable, so you'd have to check if its field satisfies the constraint `NonVoid(Abyss)`. But that brings us back to the infinite loop in comment:7. In short, I just don't see how this is possible to do in a way that's complete. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15305#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler