review: LLM context lacks enclosing control-flow — false positives on shared match arms

[ajianaz]

Observed (real case)

PR codecoradev/uteke#1084, cora review run 2 flagged a false positive:

handlers.rs:224 — the handler validates author_type up front, but only the non-contradiction branch calls set_author_type. When detect_contradiction is true, the result branch returns the id without ever setting it.

Wrong: set_author_type sits in the shared Ok(id) arm of a single match result; both remember and remember_with_contradiction(...).map(|(id,_)| id) produce id before the match, so both reach the same arm. Verified manually.

Context: run 1 on the same PR found a real adjacent bug (persist-before-validate). Correct 1/2 — the FP is a context problem, not model quality.

Root cause

build_review_prompt (src/engine/llm.rs:787) sends only the raw diff hunks (+3 context lines), file paths, static context, focus/rules. For control-flow claims ("branch X never reaches call Y") that is insufficient: the enclosing match arm, its producers, and the preceding if/else were outside the hunk. The LLM reconstructs structure from the diff shape and gets it wrong — plausible reasoning over missing evidence.

Proposal (staged)

1. Enclosing-scope context (cheap, high-value): for each hunk, include the enclosing function/block from the post-image file (tree-sitter function ranges via the index). Cap ~120 lines/function, labeled as surrounding code for control-flow verification.
2. Prompt guardrail: review rule — "Before claiming a path is unreachable or a call missing on a branch, verify branch structure in the surrounding code, not only diff context lines."
3. Later — call-graph assist: cross-check symbol reachability against the indexed call graph; annotate or downgrade confidence instead of asserting.

Acceptance

• Regression fixture: diff where a call is added inside a shared match arm fed by two producer paths — review must NOT flag branch-specific reachability without evidence.
• No token blowup: surrounding context bounded, injected only for hunks touching branching constructs (match, if, ?, early return).