control what sort of entity a deprecated pragma applies to

[igloo]

Originally reported as part of #3303.

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It's annoying not being able to control whether a type or identically named constructor is being deprecated. Consider:

data Foo = Foo ...

This is a very common idiom. But now we want to switch to smart constructors

foo :: ... -> Foo

and eventually stop exporting the constructor Foo. But we cannot specify just the constructor, only both. According to the  user guide the workaround would be to have a module that imports one but not the other, however while that's possible for the type it's not possible for the constructor.

How about

{-# DEPRECATED constructor Foo "use `foo' instead" #-}

and while we're at it, might as well have

{-# DEPRECATED type Foo "..." #-}

leaving the unqualified case meaning both as it does now.