B031: allow reusing groups after materialization

[Sonike]

After group = list(group), B031 currently flags later reads of group even though it now holds a reusable list. This recognizes explicit list(group) and tuple(group) assignments back to the same name, including annotated and chained assignments.

The assignment's right-hand side is checked before the name becomes reusable. Conditional paths are merged conservatively, so a conversion on only one branch or inside a potentially empty nested loop does not suppress genuine repeated-use warnings. Arbitrary calls such as iter(group) retain the existing behavior; this does not add alias analysis or infer the return types of helper functions.

Fixes #395.

Validation:

• Confirmed that the initial regression cases fail on the current main branch, reporting three spurious B031 diagnostics, and pass with this change.
• tox -e py313,py314: Python 3.13 — 80 passed, 1 version-specific skip; Python 3.14 — 81 passed. Both report 98% coverage.
• pre-commit run --all-files: isort, black, flake8, and rstcheck passed.
• Regression coverage includes direct/annotated/chained assignments, both and partial conditional branches, previous consumption, assignment to another name, iter(), nested loops, and deferred function bodies.

[Copilot]

🟢 Approval recommended

The implementation and new eval coverage align with the stated behavior change and appear to preserve conservative warnings in ambiguous control-flow cases.

Pull request overview

This PR updates the B031 check (groupby-group reuse) so that reassigning group = list(group) or group = tuple(group) back to the same variable is recognized as “materialized and now reusable,” avoiding false positives like #395 while still being conservative in conditionals and potentially-empty nested loops.

Changes:

• Teach B031’s AST walk to detect same-name list(...) / tuple(...) materialization assignments (including annotated and chained assignments) and treat the name as reusable afterward.
• Adjust B031 path-merging for nested loops and other constructs to avoid assuming assignments in potentially-optional/deferred execution definitely ran.
• Add eval coverage for direct/annotated/chained materialization, conditional branches, nested loops, iter(), and deferred function bodies; document the change in the changelog.

File summaries

File	Description
bugbear.py	Adds materialization detection for B031 and refines control-flow handling to avoid spurious repeated-use warnings after list(group) / tuple(group) reassignment.
tests/eval_files/b031.py	Adds regression/eval cases covering materialization and conservative merging scenarios (conditionals, nested loops, deferred bodies).
README.rst	Notes the B031 behavior change in the UNRELEASED changelog.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Lite

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[cooperlees]

Agree with copilot. Many thanks for adding this!

[cooperlees]

@copilot resolve the merge conflicts on this branch.

[Sonike]

Resolved the changelog conflict by merging the latest main in a5db492, keeping both the B031 and B028 entries. The approved B031 implementation and regression cases are unchanged.

Revalidated the combined result:

• tox -e py313,py314: Python 3.13 — 80 passed, 1 version-specific skip; Python 3.14 — 81 passed. Both report 98% coverage.
• pre-commit run --all-files: isort, black, flake8, and rstcheck passed.
• Rechecked direct list/tuple, annotated, and chained materialization assignments: four false positives on current main, none on this branch; the iter(group) reuse warning is retained.