Why can't I (or how do I) pipe to a union?

[matthew-dean]

I don't understand why this throws a type error:

pipe(
    string(),
    union([
      url(),
      literal('')
    ])
  )

It seems like a logical construction. First, pipe that it's a string, then allow a union for the next value. What's wrong with this? The type error is:

Argument of type 'UnionSchema<UnionOptions, undefined>' is not assignable to parameter of type 'BaseSchema<string, unknown, BaseIssue<unknown>> | BaseValidation<string, unknown, BaseIssue<unknown>> | BaseTransformation<string, unknown, BaseIssue<unknown>> | BaseMetadata<...>'.
  Type 'UnionSchema<UnionOptions, undefined>' is not assignable to type 'BaseSchema<string, unknown, BaseIssue<unknown>>'.
    Types of property '_types' are incompatible.

[fabian-hiller]

Here is the fix:

const Schema = v.union([v.pipe(v.string(), v.url()), v.literal('')]);