What's a big shame is that apparently it's not possible to properly use the builder with a base URL :(

• if we try to add the base URL after the input URL is built, then legitimate relative URLs are rejected (e.g. /foo + https://example.com), because /foo is not a valid URL on its own
• If we try to build the input URL with the base URL in the same time, then the parsing algorithm must be used (currently, only setters are used with a hack on line 821). Then the complication is to find some URL component that is suitable for parsing:
  • for special, full URLs: scheme + host is needed at least (e.g. https://example.com)
  • for non-special full URLs: scheme is needed at least (e.g. https://)
  • for relative URLs: the path is needed at least (e.g. /foo/bar)

And in the 2nd case, the question arises if it's ok to parse only the minimally required components and then set the rest of the components, or the whole input URL must be built and parsed all at once.