B031: don't count store-context references as generator uses

[StressTestor]

what

fixes a B031 false positive: type-annotating (or assigning to) the groupby loop variable was counted as a "use", so a single real use plus an annotation tripped "used more than once".

for _, group in groupby(items):
    group: SomeType        # annotation, not a use
    do_something(group)    # the only real use

before: B031 fires. after: clean.

why

the B031 usage-counter walked the loop body and counted every Name matching the group variable regardless of context. an annotation target (group: T) or an assignment target is a Store-context Name, not a use of the generator, so it inflated the count. count only Load-context references.

scope

this is the type-annotation / store-context case raised in the comments on #465. the original if/else case is handled separately by #557, and the two are orthogonal: if/else uses are both loads, untouched by this change, so they don't collide. real double-uses still fire B031.

tests

a case in tests/eval_files/b031.py that annotates the loop variable then uses it once, expecting no B031. it fails before the fix and passes after. full suite stays green.

Addresses #465.

[cooperlees]

Nice - Thanks for the test showing it do as you claim too!

Let's just see if copilot nit picks anything useful before merge.

[Copilot]

Pull request overview

Fixes a B031 false positive by ensuring the “group” variable from itertools.groupby() is only counted as “used” when it’s read (AST Load context), not when it’s merely a store-context reference (e.g., annotation/assignment targets). This aligns B031 with its intent—detecting multiple consumptions of the group iterator—without inflating counts due to Store-context nodes.

Changes:

• Update the B031 usage counter to count only ast.Name nodes in ast.Load context.
• Add an eval-file regression case where the loop variable is annotated and then used once (no B031 expected).
• Document the fix in the UNRELEASED changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
bugbear.py	Restricts B031 “usage” counting (and nested-loop detection) to Load-context name references.
tests/eval_files/b031.py	Adds a regression example ensuring loop-variable annotations don’t trigger B031.
README.rst	Adds an UNRELEASED changelog entry describing the B031 behavior fix.

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