Minor: Undefined behavior leads to confusing error with --index-url arg.

Environment

• pip version: 19.2.3
• Python version: 3.8
• OS: Arch Linux

I do not believe this is limited to arch, but it was discovered as such here: https://github.com/SalomonSmeke/rere/issues/5

note: as pointed out by @uranusjr below this is partially fixed in 19.3

Description

A very minor problem, borderline not a bug but simply less than ideal behavior. When using a --index-url with pip install, passing something other than a url causes a confusing error.

This can manifest like in the issue above: pip install --index-url --user https://test.pypi.org/simple/ reretrieve when trying to:
pip install --user --index-url https://test.pypi.org/simple/ reretrieve

Expected behavior

An error about how the index url is invalid, not found, or otherwise messed up.

How to Reproduce

1. pip install --index-url --user https://test.pypi.org/simple/ reretrieve

Output

$ pip install --index-url --user https://test.pypi.org/simple/ reretrieve
Looking in indexes: --user
Collecting https://test.pypi.org/simple/
  Using cached https://test.pypi.org/simple/
  ERROR: Cannot unpack file /tmp/pip-unpack-w9t0tixn/simple (downloaded from /tmp/pip-req-build-s5ub6ixa, content-type: text/html; charset=UTF-8); cannot detect archive format
ERROR: Cannot determine archive format of /tmp/pip-req-build-s5ub6ixa.

note: as pointed out by @uranusjr below in 19.3 the output will look a little different.

Edit

I just wanted to clarify that this happens with any bad text input/invalid url, not just --user and such.

$ pip install --index-url foo  https://test.pypi.org/simple/ reretrieve
Looking in indexes: foo
Collecting https://test.pypi.org/simple/
  Using cached https://test.pypi.org/simple/
  ERROR: Cannot unpack file /tmp/pip-unpack-160ive4r/simple (downloaded from /tmp/pip-req-build-8dw7kt5w, content-type: text/html; charset=UTF-8); cannot detect archive format
ERROR: Cannot determine archive format of /tmp/pip-req-build-8dw7kt5w

I dont blame you if you decide to wontfix this one, but wanted to make sure it was an intentional decision. thank you in advance for taking a look!

[chrahunt]

I’m +1 on mentioning --index-url in the warning message, assuming it doesn’t make a mess in the code.

People can have --index-url and --extra-index-url in requirements.txt which may point to directories that don’t always exist, so I would be wary of turning this into a hard failure.

[deveshks]

Thanks @uranusjr , I have created a PR considering one possible approach. Please take a look 😃