#14845: TypeInType, index GADT by constraint witness -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: TypeInType Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13895 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): But there's a significant wrinkle here. You're proposing that we change the error message to something like `Data constructor has a constraint 'cls0 a0', and thus cannot be promoted`, yes? But the place where this error message is thrown is `tcInstBinders`, which is not in any way data- constructor–specific. Indeed, as I mentioned in comment:2, this error message also occurs in the program in #13895, but that doesn't promote any data constructors whatsoever. So if we change this error message _carte blanche_, all of a sudden GHC is going to be complaining about data constructors which don't exist. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14845#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler