agent: builder model: opus max_turns: 80 budget_usd: 15 —
Score: compiler bug (false acceptance: the checker passes, rustc refuses; every lambda that composes function values), bugs-first default of docs/priorities.md. Found by b1031, narrowed by the architect at 39f8dbf6.
Part I 5.4, A function type stands wherever a type does: “the lambda moves what it captures, and a copy of the value shares the one closure”.
fn main() {
let f: fn(i64) -> i64 = fn(x) { x + 1 }
let w: fn(i64) -> i64 = fn(x) { f(x) * 2 }
println(f"{w(3)}")
}nikaia check passes; rustc: Cannot move out of f, a captured variable in an Fn closure. Lowered (nikaia lower): Kept(Arc::new(move |x| { let __nikaia_future: BoxedIn<'_, i64> = Box::pin(async move { f(x).await * 2 }); __nikaia_future })): the per-call async move moves the closure’s own f. The outer move also takes f, so f(1) after the lambda gives Borrow of moved value. A struct field lambda (Box2 { run: fn(x) { f(x) * 10 } }) is the same.
Model that works (verified, a captured list v): { let v = Arc::new(v); Kept(Arc::new(move |x| { let v = v.clone(); { ...async move { x + v.len() } } })) }. A captured hull (Shared, ADR-230 D4) is cloned in a block around the closure and once per call. A Kept is already an Arc, so a captured local of fn type wants exactly the hull treatment: let f = f.clone(); around the closure and per call.
Step 2 of #1034 (a fn parameter captured by a returned lambda, run_code in check_function.nika) is not this brief’s.
check_lambdas.nika:94 hulls_of_a_lambda (records facts.kept_hulls per lambda), :89 a_name_of_a_hull, :40 a_hull_type. Asked for a kept lambda in a let, a field, and a parameter the callee keeps.emit_core_flow.nika:217 emit_flow_hulls, :328 emit_flow_kept (clones each hull before the closure, and per call when pauses).Kept (#1031’s rule, reuse it): check_core_expr.nika:316 check_core_expr_kept_copy: a local found by lookup and not in state.run_code, or a field; a named function is no place.tests/compiler/tests/a_kept_fn_copied_from_a_name_or_field.nika.hulls_of_a_lambda, also push a name whose binding is Ty::Fn and not in run_code; rename nothing, add a doc line that a Kept is a handle like a hull.tests/compiler/tests/a_kept_lambda_calls_a_captured_fn.nika + .stdout (10, 13, 20, 2, 2, one a line), the program below; run by the architect at 39f8dbf6: three rustc errors (Cannot move out of f/g, lines 16 and 19). Commit emit: a kept lambda that calls a captured fn clones its handle (#1034), Part: #1034 (not Fixes: step 2 stays open).struct Box2 {
run: fn(i64) -> i64
}
fn inc(x: i64) -> i64 {
return x + 1
}
fn dbl(x: i64) -> i64 {
return x * 2
}
fn main() {
let f: fn(i64) -> i64 = inc
let g: fn(i64) -> i64 = dbl
let w: fn(i64) -> i64 = fn(x) { f(x) + g(x) }
println(f"{w(3)}")
println(f"{w(4)}")
let b = Box2 { run: fn(x) { f(x) * 10 } }
println(f"{b.run(1)}")
println(f"{f(1)}")
println(f"{g(1)}")
}Traced to the named cause: the let w rows (line 16). The field row (line 19) gives the same rustc message, unverified that it goes through the same kept_hulls path. If one row has a second cause, name it and its rustc message under Found:, commit the test without that row, and say so in Body:.
The test passes through nikaia test and fails on the unfixed compiler; a_kept_fn_copied_from_a_name_or_field and the hull tests (nikaia run scripts/where.nika -- kept_hulls lists them) still pass; lower-std fixpoint clean; cargo build --profile fast -p nikaia; the guard set of .claude/agents/builder.md green; pushed. Changelog: one line (a lambda kept in a let or a field that calls a function value it captures shares that value’s closure, instead of a lowering rustc refused). Stop about 15 turns before max_turns, commit what is finished and report partial.
An edit is needed outside crates/nikaia-std/src/tools/check_lambdas.nika, check_facts.nika, emit_core_flow.nika, their lowered files (and lowered.rs) and the new test; an error in another agent’s file.
Not yours, never stage: every file git status shows modified or untracked at your start, and every docs/briefs/* file except your report.
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.std or the toolchain lacks stops the work: no code around it unless an ADR or Part I-III refuses it (name it). Else Result: blocked, Found: gap + probe, Needs: owner decision (CLAUDE.md, A missing form stops the work).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.Pushed: line names the pushed sha. A report without a Pushed: line, or with a placeholder left in (TC, DISK), is not done and cuts the batch (k3a1 2026-10-29: Result: done, nothing committed)./home/user/Nikaia/docs/briefs/b1034a-kept-lambda-captured-fn.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 (redesign) for a brief with Shape: redesign and no Convert: line). 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); a process event such as another session resetting or rewriting your files is no Found:, it goes under Needs: (m3a 2026-10-28); a converter draft that came out wrong is no Found:, it is one of the costliest edits under Rs2nika:, which the converter lane reads (e12-4b 2026-10-27); 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, nor a step another commit on develop already built, shown by your test failing with that commit’s change reverted, b889 2026-10-27: name the commit in Body:, nor a test’s expected .stdout/.stderr taken from what the program really prints where the brief’s sketch of it was short or wrong, b934 2026-10-29: say so in Body:, nor a lowering that follows the ADR governing it where the brief’s expected lowering contradicts that ADR, b933 2026-10-29: name the ADR in Body:, nor a lowering other than the brief’s where rustc cannot take the brief’s, b1029 2026-10-31: say why in Body:, nor re-keying an allow-list entry (KNOWN) whose hash changed for the same program and the same reason, b-fixforward-sweep2 2026-10-28: say so in Body:, nor an existing test’s expected .stdout/.stderr updated because your change rightly changes the message it prints, b1012b 2026-10-31: name the test 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