SchemaType.validate does not allow a function to be used for a message if a custom type is not defined

[m-weeks]

Do you want to request a feature or report a bug?
bug
What is the current behavior?
SchemaType.validate does not allow a function to be sent as a message if a custom type is not defined

If the current behavior is a bug, please provide the steps to reproduce.

So if I have the following for a custom schema type:

function Email (path, options) {
  mongoose.SchemaTypes.String.call(this, path, options)
  this.validate(val => validateEmail(val), (error) => `${error.path} is invalid`)
}

An error message won't be returned when validation fails.
Now If I define a custom type:

function Email (path, options) {
  mongoose.SchemaTypes.String.call(this, path, options)
  this.validate(val => validateEmail(val), (error) => `${error.path} is invalid`, 'email')

it works as expected.

I believe the error is caused at https://github.com/Automattic/mongoose/blob/master/lib/schematype.js#L679 (if (message instanceof Object && !type) ) because apparently <function> instanceof Object is true. I would imagine typeof message === 'object' would do in its place

What is the expected behavior?
The function should be called either way, not just if there is a custom type

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node: 10.16.0
Mongoose: 5.7.11
MongoDB: 4.0.6