Works as expected :
=============================
schema = Dry::Validation.Schema do
  required(:email) { none? |  filled? }
end

schema.call({}).messages

--> {email ==> 'is missing'}

Fails:
=======

schema = Dry::Validation.Schema do
  required(:email)
end

schema.call({}).messages

---> {}   (expected same error on is missing)

Also fails:
=============
schema = Dry::Validation.Schema do
  required(:email) {}
end

schema.call({}).messages

---> Undefined method 'to_ast' for nil:NilClass in "dry-validation-0.9.5/lib/dry/validation/schema/dsl.rb:100:in `define'"