agent: builder model: opus max_turns: 80 budget_usd: 16 —

Brief: #1080, a bare `String` handed to a lent `String?` parameter is emitted without `Some`

Blocks: c1a-ledger-items.md

Score: compiler bug, a lowering rustc refuses for a form docs/builder-facts.md lists as working (a String? parameter takes a bare String); it blocks the C1a move (declared.nika declared_function(.., &target, &outer)). Location traced by reading, not sure.

Facts: Where things live, Traps already met, Commands

Issue #1080 (summary)

A String? parameter the callee only reads is lowered as &Option<String>. A bare String handed to it is emitted as &name, with no Some(..). With the body (target ?? "").len() the parameter lowers as an owned Option<String> and the call works, so the trigger is the lent-option lowering.

Probe, row a (checked on 18:40’s target/fast/nikaia, nikaia run):

fn show(target: String?, n: i64) -> i64 {
    let k = target?.len() ?? 0
    return k + n
}

fn main() {
    let name = ["ab"].join("")
    println(f"a{show(name, 1)}")
}

Lowered fn show(target: &Option<String>, n: i64) and show(&name, 1); rustc Mismatched types, expected &Option<String>, found &String. Expected output a3. Row b: in the same file, for name in ["ab", "c"] { let target = name.clone(); println(f"a{show(target, 1)}") }: same error; expected a3, a2.

Suspected location (traced by reading, not sure)

crates/nikaia-std/src/tools/check_arguments.nika, the argument loop: state.the_compiler_writes_the_reference(..) (check_references.nika :124, its one caller) writes the & and continues before any branch records FactWrap::Constructor in state.facts.nullable_args for a non-nullable value into a nullable slot; the branches at :355 (views, #598) and :405 (element reads, #655) do it for their cases only. The emitter’s wrap is emit_exprs.nika :107 (Some(..)), read where arguments are written (emit_core_args.nika). A missing or extra & at a call is the checker’s (check_references.nika); the fact carrying the wrap is nullable_args, by id_of(value).

To build

Reproduce rows a and b on the unfixed binary; keep the result under Tests:.
A non-nullable value handed to a lent T? parameter goes inside its option; any lowering rustc accepts (&Some(name) at its last use, &Some(name.clone()) otherwise). Name in Body: every function that repeats the rule (the :355 and :405 branches at least).
lower-std and its fixpoint.
Test tests/compiler/tests/lent_option_param_bare_arg.nika + .stdout with rows a and b, failing on the unfixed binary. Commit check: a bare value handed to a lent option parameter goes inside its option (Fixes #1080).

Done when

The new test fails on the unfixed compiler and passes through nikaia test --project tests/compiler --both-settings, with the tests whose names contain nullable, option, optional, argument, lent (by name); lower-std fixpoint clean; build, fmt, clippy, pre-push-check.sh; committed locally. Changelog: one line. Stop about 15 turns before max_turns: commit what is finished and report partial.

Stop and report if

An edit is needed outside check_arguments.nika, check_references.nika, emit_core_args.nika, emit_exprs.nika, their lowered twins 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.

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 (scripts/commit.nika -- REPORT does it); commit locally after each step and never push: the driver runs the complete suite and pushes (.claude/agents/builder.md, Cheap checks only; owner, 2026-10-31).
Before committing, cheap checks only: cargo build --profile fast -p nikaia, lower-std and its fixpoint when a tools/*.nika changed, cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, the tests the brief names and the ones your change touches. No gate.nika.
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.
A form Nikaia, 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: architect decision (CLAUDE.md, A missing form stops the work).
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.
Time box: no cause after about 40 turns, or going in circles: stop, Result: partial with what you ruled out, the probe, and the suspected file and function (.claude/agents/builder.md).
Write the report, then commit with target/fast/nikaia run scripts/commit.nika -- REPORT, which writes Pushed: local <sha> into it. 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).

Report (at most 15 lines, to exactly `/home/user/Nikaia/docs/briefs/bugs/b19-1080-lent-option-bare-arg.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

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Brief: #1080, a bare String handed to a lent String? parameter is emitted without Some