Make it possible to deprecate a method instantiation of a typeclass instance

Consider:

module A where

data Foo = Foo

instance Eq Foo where
  -- {-# DEPRECATED (==) "Deprecated for no reason as well" #-}
  _a == _b = True


{-# DEPRECATED (==.) "Deprecated for no reason" #-}
(==.) :: Foo -> Foo -> Bool
(==.) _a _b = True

Deprecating (==.) is possible, but it's not possible to deprecate (==) of the Eq Foo instance.

I'd be useful for my use-case of finding out where Ord Unique is used, as these would be a potential sources of non-determinism. Currently the best I can do is to remove the instance, get a compile error, suppress it by fixing up the code and repeat for every affected file.

I imagine it would also be useful if a method turned out to be a bad idea for a particular type and the library author tried to phase it out. It could be that one method is implementable, but has terrible performance.

For my use-case I would be happy with instance level granularity.

Related (but not quite the same):

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Trac field	Value
Version
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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CC	simonmar
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Make it possible to deprecate a method instantiation of a typeclass instance