agent: builder model: opus max_turns: 110 budget_usd: 22 —
T?-typed let whose arm is a proved name (Fixes #893)Score: bugs lane (docs/priorities.md, bugs-first default): nikaia check passes, rustc refuses; found by the #888 builder.
fn f(b: String?, c: String) -> String? {
let s: String? = if b != null { b } else { c }
return s
}
fn main() { println(f("x", "y") ?? "n") }nikaia check passes; rustc: Mismatched types at the b arm (b is written as the proved String while the let wants Option<String>; the c arm also needs Some(..)). Fails before and after #888 (3b77d7c), which handled only a let whose type is owned whole and not optional.
T? position (a let s: T? = ..., an assignment to a T? place, a return from a -> T? function) is written as the option itself (b, not b.unwrap() re-wrapped), and a non-optional arm is wrapped in Some(..). Find where return of the same form is lowered and use the same path (nikaia run scripts/where.nika -- narrowed_reads_given_back). Test: tests/compiler/tests/a_proved_arm_bound_at_an_optional_let.nika + .stdout, forms let, assignment, return, with String and a struct.Rules: ADR-382 (a proved name handed back whole), ADR-352.
The new test; nikaia test tests/compiler --both-settings; it filtered on narrow nullable handed_over; the guard set of .claude/agents/builder.md; clippy clean. Commit with Fixes: #893; Changelog: one sentence.
The fix needs a rule ADR-382 / ADR-352 do not give (an open design choice); an error in another agent’s file.
Read docs/builder-facts.md.
crates/nikaia-std/src/tools/emit_core_calls.nika, its lowered/emit_core_calls.rs, crates/rs2nika/convert/**. Never stage them; if lower-std rewrites lowered/emit_core_calls.rs, leave it out of your commit and say so in Body:.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).cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, and the gate of .claude/agents/builder.md (#852).CHANGELOG.md, badges; no new NK code. New tests in Nikaia only; never the full suite. No foreground sleep, no polling; scripts in files.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./home/user/Nikaia/docs/briefs/b893-optional-let-proved-arm.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