## Motivation In a software project, some instances should maybe never be implemented, for instance ``Eq`` on very large data structures. I can ensure this by giving a dummy implementation like: ```haskell instance Eq D where (==) = undefined ``` However, this gives me only a runtime error, but I wish to get a compile-time error if trying to use equality on `D`, just as if I hadn't implemented an instance in the first place. I could get a compile-time alert if it was possible to attach WARNING or DEPRECATED pragmas to instances or method implementations. ## Proposal Allow WARNING and DEPRECATED on instances, e.g.: ```haskell instance Eq D {-# WARNING: "No equality on D!" #-} where (==) = undefined ``` The behavior should be similar to a missing implementation of `Eq D`, i.e., a warning generated during type-checking. Maybe it is possible to throw a hard error also? (I did not find an ERROR pragma.)