agent: builder model: opus max_turns: 100 budget_usd: 20 —

Brief: a name proved by a guard in a tuple arm with `null` or a literal beside it (Fixes #889)

Score: bugs lane (docs/priorities.md, bugs-first default): nikaia check passes, rustc refuses (E0308); found by rs2nika lane L building #879.

Issue text (#889)

fn g2(a: String?, b: String?) -> String {
    return match (a, b) {
        (x, null) if x != null => x,
        else => "none",
    }
}
fn main() { println(g2("a", null)) }

nikaia check passes. nikaia run fails in rustc with E0308: x is lowered as x.as_ref().unwrap(), a &String, where the result is a String handed back by value. The arm (x, 2) if x != null => x fails the same way. (x, _) if x != null => x works, so the fault is in how the arm’s pattern is lowered when another position holds null or a literal: the binding is then by reference. Related to d89aba7 (a proved name handed back whole is given up) and #888, but a separate path.

Locations

crates/nikaia-std/src/tools/check_arm_leaves.nika:394 a_narrowed_read_given_up, :419 narrowed_reads_given_back (d89aba7: narrowed_reads +8 marks a proved name given up, x.unwrap()).
Match lowering: crates/nikaia-std/src/tools/emit_core_match.nika (emit_match_subject :93; a tuple subject with a null/literal position is likely matched by reference, match (&a, &b) or .as_ref(), so the binding is &String?); patterns emit_pattern.nika.
Find first, with nikaia run --keep or the lowered Rust, which of the two differs between (x, _) and (x, null): the subject’s form or the narrowed read.

To build

Emit: a name the guard proves, handed back whole from an arm whose pattern has null or a literal at another position, is given up by value (x.unwrap(), or bound by value), as in the (x, _) case. Test tests/compiler/tests/a_guard_proved_name_beside_null_given_back.nika + .stdout: the probe, the (x, 2) form with (String?, i64), and a struct in place of String.

Done when

The new test (fails on the unfixed compiler); nikaia test tests/compiler --both-settings; it filtered on handed_over text_tiers typecheck narrow; the guard set of .claude/agents/builder.md; clippy clean. Commit with Fixes: #889; Changelog: one sentence.

Stop and report if

Binding by value would move a subject that is read after the match (then blocked with the lowered forms); an error in another agent’s file.

Read docs/builder-facts.md.

Not yours (another lane’s uncommitted files in the checkout, 2026-10-27): crates/rs2nika/convert/**. Never stage them.

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/b889-guard-tuple-arm-by-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, and for a move Rs2nika: (the kept N of M lines of rs2nika --score, then the three costliest hand edits; none for a task that moves no Rust). 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

This site is open source. Improve this page.

Brief: a name proved by a guard in a tuple arm with null or a literal beside it (Fixes #889)