avoid spurious empty-constraint conflict with duplicates across extras

[dimbleby]

I think this is probably right - the testcase is good anyway, and the fix looks likely...

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When duplicate dependencies on the same package had different extras (e.g. the package appeared in multiple project.optional-dependencies entries and was also required with an extra by a dependency group), _resolve_overlapping_markers synthesised a 'not required' empty-constraint dependency for the leftover marker space of each subgroup. That synthetic dep conflicted with the sibling subgroup, causing 'depends on both X (*) and X ()' resolution failures.

Make the leftover-marker-space placeholder opt-in via a new cover_leftover_marker_space flag, and only enable it for the single-complete-name caller that actually needs it.

Fixes #10447

[sourcery-ai]

Hey - I've left some high level feedback:

• The new cover_leftover_marker_space flag introduces a subtle behavioral fork in _resolve_overlapping_markers; consider either documenting its semantics and intended callers more explicitly in the docstring or splitting into two dedicated helpers to avoid boolean-flag-driven behavior.
• The comment in the non-single_complete_name branch explains why cover_leftover_marker_space must be false there; it might be helpful to add a short in-code assertion or helper naming to enforce that assumption and make future regressions around leftover marker space harder to reintroduce.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `cover_leftover_marker_space` flag introduces a subtle behavioral fork in `_resolve_overlapping_markers`; consider either documenting its semantics and intended callers more explicitly in the docstring or splitting into two dedicated helpers to avoid boolean-flag-driven behavior.
- The comment in the non-`single_complete_name` branch explains why `cover_leftover_marker_space` must be false there; it might be helpful to add a short in-code assertion or helper naming to enforce that assumption and make future regressions around leftover marker space harder to reintroduce.

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[dosubot]

Related Knowledge

1 document with suggested updates is ready for review.

Python Poetry

CHANGELOG /poetry/blob/main/CHANGELOG.md — ⏳ Awaiting Merge

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