agent: builder model: opus max_turns: 100 budget_usd: 20 —
let-bound if is not given up whole (Fixes #888)Score: bugs lane (docs/priorities.md, bugs-first default; #888 is a checker-passes/rustc-fails bug, found by the #879 builder; reproduced by the architect on develop 993664a).
d89aba7 (ADR-382, #879) makes a name proved not null the value itself when the function hands it back whole, by return or as its tail (check_arm_leaves.nika: a_narrowed_read_given_back, narrowed_reads_given_back). A value bound by let is not covered. Probe on 993664a:
fn t2(n: i64) -> String {
let b: String? = if n > 0 { f"i{n}" } else { null }
let s: String = if b != null { b } else { "none" }
s
}
fn main() { println(t2(3)); println(t2(0)) }nikaia check passes; nikaia run fails in rustc, “Mismatched types” at the b in the then arm (lowered b.as_ref().unwrap()). The match form let s: String = match b { null => "none", x => x } already runs (control). A lent parameter a: String? in the same forms is refused with NK1103/NK1104: that is #882’s territory (main-session lane), do not touch it.
crates/nikaia-std/src/tools/check_arm_leaves.nika:342 (a_narrowed_read_given_back, :356, narrowed_reads_given_back, narrowed_reads_given_back_in, added by d89aba7); callers check_core_stmt.nika (the return case) and check_function.nika (the tail, if !tail_view). The let statement’s check is in check_core_stmt.nika (find the Stmt::Let arm with nikaia run scripts/where.nika -- NAME). Test of d89aba7: tests/compiler/tests/a_proved_value_handed_back_whole_is_given_up.nika.
let s = if ... (type inferred) and with an assignment s = if b != null { b } else { "x" } to a let mut s: String. Controls: the match form above; let s: String = if b != null { b.clone() } else { "none" }.let (and an assignment) whose type is owned text or another owned non-copied type is given up as a return is: call narrowed_reads_given_back on it. The read is a move of b: the same rule as let s: String = t for a plain String local t applies (probe what the checker does when b is read after the let, and make the narrowed form do the same; if that form is refused, the narrowed one is refused with the same code). A view-typed let (ref String, or a local the checker makes a view) stays as today.tests/compiler/tests/a_proved_value_bound_by_let_is_given_up.nika with .stdout: every form of step 1 and the controls. Last commit Fixes #888.The new test; nikaia test tests/compiler --both-settings; the guard set of .claude/agents/builder.md; lower-std unchanged or changed only where a tools file used the form (say which under Tests:). Changelog: one line.
An open design choice (for example: the move makes a later read of b refused where today it compiles); an error in another agent’s file.
Read docs/builder-facts.md.
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/b888-let-bound-proved-name.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