Have you had a look at the source code?

// Marshaling interface implementations.
// Would like to implement MarshalText/UnmarshalText but that will change the JSON representation of URLs.

func (u *URL) MarshalBinary() (text []byte, err error) {
        return []byte(u.String()), nil
}

func (u *URL) UnmarshalBinary(text []byte) error {
        u1, err := Parse(string(text))
        if err != nil {
                return err
        }
        *u = *u1
        return nil
}

In particular, read the commit that added this code - 6595709. It seems like adding what you suggest would break existing code. Perhaps this can be left for Go2, or for any other opportunity where backwards compatibility can be broken in this package.

Duplicate #10275

Thanks, @bontibon. It indeed makes sense to group all of these under one umbrella issue for Go2.

It seems like adding what you suggest would break existing code.

Why? I don't understand.

Would like to implement MarshalText/UnmarshalText but that will change the JSON representation of URLs.

Only implementing UnmarshalText wouldn't change the JSON representation of URLs.

What is wrong with only implementing the parsing portion?