Running pip-compile with a dev-requirements.in that adds an extra to a library has inconsistent output

[arthurio]

When you define a requirements.in file that has a library with no [extra] and use the requirement.txt output as input for a dev-requirements.in file that adds an [extra], the output dev-requirements.txt will inconsistently have either the [extra] at the end of the library or not...

Environment Versions

1. MacOS 11.5.2 (but also happens on linux)
2. Python version: 3.7.11, 3.8.7, 3.9.1
3. pip version: 21.2.4
4. pip-tools version: 6.2.0

Steps to replicate

I have created a reproducible demo:

1. Minimal library repo: https://github.com/arthurio/pip-tools-bug-lib
2. Bug repro repo: https://github.com/arthurio/pip-tools-bug-extra-repro

In the bug repro repo, I have a script called pip-compile.sh. If you remove the .txt files and run the script twice, you'll see that the first time dev-requirements.txt contains git+https://git@github.com/arthurio/pip-tools-bug-lib#egg=my-lib[test]
but the second time it contains git+https://git@github.com/arthurio/pip-tools-bug-lib#egg=my-lib.

You can see that from the 2 consecutive commits:

1. arthurio/pip-tools-bug-extra-repro@63cefce#diff-43470c4f399e798afc682f0bf6c690eef5f47ee25445904a066e4c3314f3a787R11
2. arthurio/pip-tools-bug-extra-repro@33f8d6f#diff-43470c4f399e798afc682f0bf6c690eef5f47ee25445904a066e4c3314f3a787R11

Expected result

I would expect pip-compile to always keep the [extra] flag.

Actual result

pip-compile strips out the [extra] flag the second time it's ran.

[arthurio]

Also, the same happens with just a library name (not using a git url)

[AndydeCleyre]

This might be a dupe of #1451

[arthurio]

@AndydeCleyre You are right, sorry about that!

[AndydeCleyre]

No worries! Thanks for the issue; your documented case helps describe the problem.