agent: builder model: opus max_turns: 110 budget_usd: 22 —

Brief: a value moved in an arm, bound by a `let` (Fixes #892)

Score: bugs lane (docs/priorities.md, bugs-first default): a soundness gap, the checker passes a use after move that rustc refuses; found by the #888 builder.

Issue text (#892)

Found by the #888 builder (report docs/briefs/b888-let-bound-proved-name.md.report.md), reproduced by the architect on 3b77d7c.

Probe

struct P { name: String }
fn main() {
    let t = P { name: "a" }
    let c = true
    let s: P = if c { t } else { P { name: "b" } }   // also: match c { true => t, false => ... }
    println(s.name)
    println(t.name)
}

nikaia check passes; nikaia run fails in rustc: Borrow of moved value: t. The control let s: P = t; println(t.name) is NK2105 (t was bound to s by a let). The proved form (let s: P = if p != null { p } else { ... }, then a read of p) does the same.

Cause

hands_over (crates/nikaia-std/src/tools/check_arm_leaves.nika:389) returns at once unless the value is a name or a field path, so the tails of an if/match/block value are never handed over. narrowed_reads_given_back (same file, :356) already walks those tails (if with else, block tail, each match arm body).

To build

hands_over walks the tails of Expr::If (with an else), Expr::Block and Expr::Match the way narrowed_reads_given_back does, handing over each tail that is a name or field path (one arm moving is enough for NK2105 after the if). Test: tests/compiler/tests/a_value_moved_in_an_arm_is_handed_over.nika + .stderr (if form, match form, proved form: each NK2105), and a passing control where the moved name is not read again, with .stdout.
Check every caller of hands_over (10 in 7 files, nikaia run scripts/where.nika -- hands_over): function arguments and struct fields given an if value get the same rule; lower-std re-lowers the touched files, and the guard set of tools/*.nika still checks (a tools file that now hits NK2105 is a finding in that file: fix it with .clone() only where the value is really read again, and say so in Body:).

Rules: ADR-094 D4, ADR-293 D29 (a rename moves). A text (String) moved in an arm and read later already runs (the checker copies it); keep that behaviour.

Known facts

Read docs/builder-facts.md. The let site that calls hands_over: crates/nikaia-std/src/tools/check_core_stmt.nika:519. The commit says Fixes #892.

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Read docs/builder-facts.md first. develop in the main checkout; git status at start: other agents’ uncommitted files are not yours, never touch or stage them.
git add <files> only; commit after each step; git push origin develop fast-forward, no rebase (rejected: report Pushed: rejected, go on committing).
Before pushing: cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, and the gate of .claude/agents/builder.md (#852).
No version lines, CHANGELOG.md, badges; no new NK code. New tests in Nikaia only; never the full suite. No foreground sleep, no polling; scripts in files.
Disk: NIKAIA_CACHE_LIMIT=2G; at most two clones with target/ in the machine; delete your own clone’s target/ and target/release/nikaia-scratch at the end; report GB freed.

Report (at most 15 lines, to exactly `/home/user/Nikaia/docs/briefs/b892-arm-moved-value.md.report.md`)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word alone on that line: Result: done, never a summary after it, which cuts the batch, b869a 2026-10-27; Owner: is a question for the owner or none, never a role), Files, Commit, Body (optional, one paragraph for the commit message), Fixes, Pushed, Tests, Gate (green|red), Deviation, Found, Needs, Changelog, Owner, Disk freed. none where nothing, and then the bare word: Deviation: none (a note) cuts the batch (batch.nika compares exactly); a note goes into the commit message. Found: is only for a defect outside your change (it cuts the batch); how you solved your own step goes into the commit message. Deviation: is only a step of the brief built otherwise than written or not built (a choice the brief left to you, such as keep what X needs and say so, is no deviation: it goes into the commit message; nor is building two steps in one commit when every step is built, e12-3a 2026-10-27: say so in Body:); how you verified (e.g. a test not run against the unfixed compiler) goes under Tests:. A noop says why under Tests: (what was probed), not under Deviation:. Final answer, one line: done <sha>, report <path> (or BLOCKED: ... / PUSH REJECTED ...).

Attribution: Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_01GFeMqxRbtUMiGVyDMxsvEq

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Brief: a value moved in an arm, bound by a let (Fixes #892)