pip-compile incorrectly indicates packages were not pinned

[BenjaminPelletier]

Pinning setuptools causes a warning indicating that setuptools was not pinned.

Environment Versions

1. OS Type Linux
2. Python version: $ python -V 3.11.9
3. pip version: $ pip --version 25.0
4. pip-tools version: $ pip-compile --version 7.4.1

Steps to replicate

1. Create requirements.in with the following content:

flask==3.1.0

2. Attempt to generate requirements.txt: pip-compile --generate-hashes --output-file=requirements.txt requirements.in
3. Observe success and no warning (as expected)
4. Determine current working version of setuptools: pip list | grep setuptools (mine is 75.8.0)
5. Pin setuptools in requirements.in by changing its content to:

flask==3.1.0
setuptools==75.8.0

6. Attempt to generate requirements.txt: pip-compile --generate-hashes --output-file=requirements.txt requirements.in

Expected result

Success, or at least warnings that make sense.

Actual result

# WARNING: The following packages were not pinned, but pip requires them to be
# pinned when the requirements file includes hashes and the requirement is not
# satisfied by a package already installed. Consider using the --allow-unsafe flag.
# setuptools
The generated requirements file may be rejected by pip install. See # WARNING lines for details.

This warning is extremely surprising since the change that elicits this warning is pinning the very package that the message claims was not pinned.