#15431: Coercible and Existential types don't play nicely -------------------------------------+------------------------------------- Reporter: NioBium | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Roles Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14333 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I'm not all that surprised here. GHC has no way of decomposing `Coercible Int (t a)`, and so it just remembers that constraint in case it needs to prove exactly `Coercible Int (t a)`. In this case, it's trying to prove `Coercible (t a) Int`, and so it gives up. I suppose we special-case the lookup to include looking for a symmetrical constraint, but there will always be incompleteness lurking around the corner. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15431#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler