Does first slash is compulsory in URL?

Question

Today we had an arguing with colleague about if that would be a valid URL http://domain.com?some_param=1. The RFC that defines URL structure does not explicitly states that that is invalid structure. According to RFC 1738, part 3.3

If neither <path> nor <searchpart> is present, the "/" may also be omitted.

However, at the end of part 3.1 of same document it says

Note that the "/" between the host (or port) and the url-path is  NOT part of the url-path.

Checking up with curl revealed some interesting thing

$ curl -v -I http://stackoverflow.com?test
* Rebuilt URL to: http://stackoverflow.com/?test
...

So, question is

Is this is a valid URL or not? http://domain.com?some_param=1

Answer 1

Section 3.3 defers to section 3.1 for host and port, but the rest is as defined in that section. Thus, the slash after port is required if either of path or searchpart is present.

Nonetheless tools may attempt to extract a valid URL even with the omission of this slash, since one of the precepts of various ideologies is to be liberal on input and conservative on output.