Style suggestion: punning syntax for label arguments with casting coercion

[mbarbin]

This is a style suggestion to support the label punning when the labeled argument is casted with the :> syntax.

Current formatting
Please copy-paste your code and the formatting that is currently applied to it.

let f ~x = x
let coerce_punning x = f ~(x : int)
let cast_coerce_punning x = f ~x:(x :> int)

The coerce_punning works already. The request is about the second item.

Describe the formatting you'd like
A clear and concise description of what you want to happen.

let cast_coerce_punning x = f ~(x :> int)

Additional context

This is quite minor. This is motivated for more consistency. I don't know off-hand from which ocaml-version this punning syntax was supported. Thank you!