Incorrect handling of empty host in absolute URL

[kenballus]

Describe the bug

Absolute URLs are permitted to have empty hosts in RFC 3986.

Relevant grammar rules:

   host          = IP-literal / IPv4address / reg-name
   reg-name      = *( unreserved / pct-encoded / sub-delims )

Thus, a URL like a://:1 conforms to the standard.
However, yarl rejects this URL.

urllib3, CPython urllib, rfc3986, furl, and hyperlink all correctly handle this situation.

To Reproduce

Try running the following snippet:

>>> yarl.URL("a://:1")

Expected behavior

The parse should have succeeded, resulting in URL('a://:1').

Logs/tracebacks

This is the output from the snippet:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.10/site-packages/yarl/_url.py", line 163, in __new__
    raise ValueError("Invalid URL: host is required for absolute urls")
ValueError: Invalid URL: host is required for absolute urls

### Python Version

```console
$ python --version
Python 3.10.9

multidict Version

$ python -m pip show multidict
Name: multidict
Version: 6.0.4
Summary: multidict implementation
Home-page: https://github.com/aio-libs/multidict
Author: Andrew Svetlov
Author-email: andrew.svetlov@gmail.com
License: Apache 2
Location: /home/bkallus/fuzzing/url_differential_fuzzing/url_fuzz_env/lib/python3.10/site-packages
Requires:
Required-by: yarl

yarl Version

$ python -m pip show yarl
Name: yarl
Version: 1.8.2
Summary: Yet another URL library
Home-page: https://github.com/aio-libs/yarl/
Author: Andrew Svetlov
Author-email: andrew.svetlov@gmail.com
License: Apache-2.0
Location: /home/bkallus/fuzzing/url_differential_fuzzing/url_fuzz_env/lib/python3.10/site-packages
Requires: idna, multidict
Required-by:

OS

Arch Linux

Additional context

No response

Code of Conduct

• I agree to follow the aio-libs Code of Conduct

[webknjaz]

Hi, if you could send a PR with just a test that could then be marked as xfail, it'd be useful.

[kenballus]

Hi, if you could send a PR with just a test that could then be marked as xfail, it'd be useful.

Done!

I'm not too familiar with the codebase, but I imagine the fix is to delete lines 187 and 188 in yarl/_url.py:

187:                if host is None:
188:                    raise ValueError("Invalid URL: host is required for absolute urls")

A host is always permitted to be empty, unless scheme-specific rules say otherwise.