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

Brief: #655 step 2b, `if c { null } else { given[1] }` under `ref E?`; then `check_arguments_at` goes

Score: bugs lane (docs/priorities.md): a false refusal (NK1103) that keeps a detour in our own code (check_arguments_at in check_arguments.nika, kept in 2e1ba5d for this reason); small, one area. Runs after step 2a (b655-element-view-slots.md).

Issue text (#655, item 1, step 2)

if c { null } else { given[1] } is NK1103: the branch is typed E?. To build 2: a view of an element as … an if branch under ref E?: it lowers to Some(&..) and is accepted. One test per probe. After step 1: check_arguments drops check_arguments_at. Decided by ADR-354 D1 (an arm given[0] under ref Expr? is a view of the element); null stays None.

Probes (`struct E { n: i64 }`; architect ran them on 2e1ba5d’s binary)

(a) fn f(given: ref Vec[E], c: bool) -> i64 { let y: ref E? = if c { null } else { given[1] }; return (y ?? return 0).n } (one statement a line), main calls f(g, false): NK1103 This value is E?, but the let declares ref E?. Expected 2; with true, 0.
(b) the same as a match with a null arm, and as a function result -> ref E? whose last value is that if.
(c) control, already fine: if c { t } else { given[1] } with t: ref E; and let mut y: ref E? = null / y = t.

Then: the detour

crates/nikaia-std/src/tools/check_arguments.nika: check_arguments_at 19-ish (comment says it exists because a ref Expr? is not built from an if) is replaced at its one call (~line 219, given: check_arguments_at(given, at)) by the if it stood for, and deleted. If it still fails for another reason, that is a Found: with the probe.

Locations

crates/nikaia-std/src/tools/check_arm_leaves.nika: the arm walk ~595-645 (check_core_stmt_element_view at ~616 under what == "let"; ends_in_null, note_mixed 104); where an if’s type is joined from null and an element arm before the let’s type is asked: find with nikaia run scripts/where.nika -- NAME from there.
crates/nikaia-std/src/tools/check_core_stmt.nika check_core_stmt_element_view 330.
Generated files: crates/nikaia-std/src/tools/lowered/*.rs (docs/builder-facts.md).
Tests: new tests/compiler/tests/a_null_arm_beside_an_element_view_is_an_optional_view.nika + .stdout (a and b).

Done when

Probes (a)-(b) print as stated, (c) unchanged, check_arguments_at is gone. nikaia test tests/compiler --both-settings passes, plus it filters element_views, views_kept, nullable, match_arms. Commit A null arm beside a view of an element is an optional view (Part of #655); no Fixes (step 5 remains). Changelog line: yes.

Stop and report if

An open design choice: Result: blocked, the question under Owner:.

Read docs/builder-facts.md.

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/b655-null-arm-view.md.report.md`)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word first; Owner: is a question for the owner or none, never a role), Files, Commit, 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; 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: #655 step 2b, if c { null } else { given[1] } under ref E?; then check_arguments_at goes