.default() should apply default if wrapped schema transforms to undefined

[jedwards1211]

This may seem strange, but it's because I've been dealing with AWS CloudFormation parameters, where you can only have empty strings, not null or undefined values...

I was trying to do

// helper used in place of `.optional()` for my schema for parsing CloudFormation metadata
function optional<T extends z.ZodTypeAny>(schema: T) {
  return z
    .union([z.literal('').transform(() => undefined), schema.optional()])
    .optional()
}

const parameter = optional(z.string()).default('test')

I expected parameter.parse('') to be 'test' but instead it's undefined. This violates the output type of .default('test'), so I think even if the input is defined, ZodDefault should check that the parse output is too.

[jedwards1211]

Here we're seeing the consequence of designing ZodDefault to change the input to a default value if it's undefined, rather than just changing the output value if it's undefined.

To guarantee that ZodDefault does produce the correct output type, I had to:

• Re-parse on the default value if parsing a non-null value output undefined
• In the worst case, even parsing the default value outputs undefined -- I had to add an error for this case