Inconsistency in _ -> - normalization (extras)

[Conchylicultor]

Description

I'm using a PEP 621 toml using flit 3.5 & pip 21.3.1:

My toml has self-refering optional-dependencies:

[build-system]
requires = ["flit_core >=3.5,<4"]
build-backend = "flit_core.buildapi"

[project]
name = "etils"

[project.optional-dependencies]
array_types = ["numpy"]
all = ["etils[array_types]"]

When running pip install -e .[all], this logs:

WARNING: etils 0.2.0 does not provide the extra 'array-types'

It looks like "etils[array_types]" gets normalized to [array-types], but not array_types = ["numpy"].

Manually replacing array_types = ["numpy"] -> array-types = ["numpy"] solve the issue.

But this behavior feels inconsistent.

I first posted in pypa/flit#503 but from the Flit authors, it seems this is a pip issue. It seems Flit generate the following metadata:

Requires-Dist: installer[array_types] ; extra == "all"
Requires-Dist: numpy ; extra == "array_types"
Provides-Extra: all
Provides-Extra: array_types

Expected behavior

No response

pip version

pip 21.3.1

Python version

3.8

OS

Linux

How to Reproduce

• Create a project, with roughly the same pyproject.toml as above.
• pip install .[all]
• See the warning about extra names.

Output

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[uranusjr]

Moving discussion to https://discuss.python.org/t/7614 sine this is ultimately a bug in the specification and pip can only follow what the rules allow it to. pip would receive a fix automatically once this gets resolved at the PEP level and implemented in packaging.

[Conchylicultor]

Independently of which specification get choosen, Shouldn't pip be at least internally consistent ?