Failing to synthesize Finset Bind instance

Stream: new members

[Jesse Slater]

I don't understand why this doesn't work:

import Mathlib
def foo (s : Finset ℕ) (f : ℕ → Finset ℕ)
 : (Finset ℕ)
 := s >>= f  --failed to synthesize Bind Finset

My expectation is that since Finset has a Monad instance and Monad extends Bind, then I should be able to use >>= notation with Finsets. It looks like the notation is being used at docs#Finset.bind_def, so I don't understand why it doesn't work here.
I know I can use Finset.sup, but I'm curious what I am missing.

[Eric Wieser]

You can only use it with open scoped Classical