date().required() does not validate correctly within when()

[pcorey]

Given this code:

const { date, mixed, object, string } = require('yup');

const schema = object().shape({
    foo: string(),
    date: mixed().when('foo', {
        is: 'bar',
        then: date().required()
    })
});

console.log(schema.validateSync({ foo: 'boo' }));
console.log(schema.validateSync({ foo: 'bar', date: '123' }));
console.log(schema.validateSync({ foo: 'bar', date: 'hi' }));

The first two calls to schema.validateSync pass, as expected. The third call that validates { foo: 'bar', date: 'hi' } also passes validation, which seems wrong. 'hi' is not a valid date, and date() wouldn't accept this value outside of a while() block.

I'm on yup@0.26.6.

[jquense]

Seems like a bug!