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:

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.:

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.)