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

Brief: a struct field at a `T?` position narrows after its `null` arm (Fixes #886)

Score: bugs lane (docs/priorities.md, bugs-first default; #886 is the oldest of the open ready checker bugs #886 #888 #889 #890 #891).

Issue #886 (copied)

Found by the #879 builder. #879 assumed that struct fields already narrow per position. They do not.

struct F { name: String?, n: i64 }
fn len(v: F) -> i64 {
    match v { F { name: null, .. } => 0, F { name, .. } => name.len() }
}

The checker passes this program. rustc then refuses it (“Method len is private”): name stays an Option<String> in the lowered code. ADR-352 D1 is written for patterns in general, so a struct field gets the per-position rule a tuple position has.

To build

Checker: in a match, a bare field binding at a T? field binds the T once an earlier unguarded arm has null at that field and covers the other fields. This is the rule set_positions already applies to tuples.
Emit: such a field pattern is written Some(name). A variant or literal at that field is matched against the T.
Tests in tests/compiler/tests/: the probe above (also F { name: x, .. } with a rename, and a field after null of an earlier arm that names other fields ..), and a field without its null arm that stays T? (NK1125; its test file: nikaia run scripts/where.nika -- NK1125).

Locations

crates/nikaia-std/src/tools/check_core_flow.nika: check_core_flow_same_part :617, check_core_flow_covers_null_at :652, check_core_flow_set_tuple :673 (writes state.facts.set_positions :714), check_core_flow_parts :720 (the bindings of a pattern). Build the struct case beside the tuple one; key the fact so the emit can tell the field (a position = the field’s index in the struct’s declaration order is enough, if the emit has it; say which in the commit).
crates/nikaia-std/src/tools/check_facts.nika:275 set_positions.
crates/nikaia-std/src/tools/emit_core_match.nika:342 reads set_positions for tuple arms; the struct pattern writer is in the same file.
Tools files changed are re-lowered (docs/builder-facts.md, lowering).

Facts

Read docs/builder-facts.md.

Done when

The probe fails before, passes after; new tests pass at both settings; nikaia test --project tests/compiler filters narrow, null, match, tuple green; the it filter for the NK1125 test green. Last commit says Fixes #886.

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/b886-struct-field-narrowing.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, 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

This site is open source. Improve this page.

Brief: a struct field at a T? position narrows after its null arm (Fixes #886)