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Brief: #1023, three lowering defects of fn parameters handed on (blocks K3a part 2)

Score: bug before move; it blocks the self-hosting row K3a (k3a2 stopped on it, run 4). The whole issue, copied:

Found by k3a2 (K3a part 2, run 4; docs/briefs/k3a2-comptime-compute.md.report.md). Blocks K3a part 2 (comptime_computed / comptime_worked_out_ahead, draft docs/briefs/k3a2-comptime-compute.run4.patch).

Problem and probes (run by the architect at 32080dd6)

  1. Send + Sync resolves to tools’ own enum Sync. A kept callback in a crate that overlaps lowers Kept<dyn Fn(..) -> R + Send + Sync> (emit_core_types.nika:183 and :191); inside tools, ty.nika:693’s pub enum Sync shadows the trait: E0404 at lowered/comptime_tree.rs:473. The emitter writes std::marker::Send + std::marker::Sync.
  2. A let lambda that shadows the fn parameter it reads is handed on unlent. 
    fn alt(name: String) -> i64? sync { return name.len() }
fn use_it(known: fn(ref String) -> i64? sync) -> i64 sync { return known("ab") ?? 0 }
fn outer(known: fn(ref String) -> i64? sync) -> i64 sync {
    let known = fn(name) { known(name) ?? alt(name) }
    return use_it(known)
}
fn main() { println(f"{outer(fn(name) { null })}") }

    rustc E0308 “expected &_, found closure”; should print 2. Without the shadowing (let more = ...; use_it(more)) it writes &more. The emitter takes the name for the parameter, not for the local that shadows it.

  3. A function in a call cycle that hands a new lambda to its own fn parameter instantiates itself without end. Direct (deep(n - 1, more) from deep) and mutual: 
    fn alt(name: String) -> i64? sync { return name.len() }
fn ping(n: i64, known: fn(ref String) -> i64? sync) -> i64 sync {
    if n == 0 { return known("a") ?? 0 }
    let more = fn(name) { known(name) ?? alt(name) }
    return pong(n - 1, more)
}
fn pong(n: i64, known: fn(ref String) -> i64? sync) -> i64 sync { return ping(n, known) }
fn main() { println(f"{ping(2, fn(_name) { null })}") }

    “Reached the recursion limit while instantiating ping::<{closure}>”; should print 1. ADR-277 D11 writes a run sync parameter impl Fn(A) -> R; each new closure is a new type, so in a cycle the instantiation never ends. Handing the same parameter on (deep(n - 1, known)) already works (prints 3) and must keep working.

Decision (architect, a lowering defect: the programs are valid, D11 did not foresee the cycle)

ADR-277 D11 gains one sentence, committed with the build: A run sync parameter of a function that lies on a cycle of the call graph is written &dyn Fn(A) -> R: impl Fn would make each new lambda a new instantiation, without end. The cost is one indirect call per call of the parameter, only on cycles. A pausing parameter on a cycle is not decided here.

Locations

To build

  1. Defect 1: both crossing strings fully qualified. Check: after lower-std, grep -n "+ Send + Sync" crates/nikaia-std/src/tools/lowered/*.rs is empty. Commit.
  2. Defect 2: an argument naming a local that shadows a code parameter is lent as a local. Test tests/compiler/tests/a_shadowing_lambda_is_lent.nika (+ .stdout 2): probe 2. Commit.
  3. Defect 3 and the ADR-277 D11 sentence. Tests a_lambda_handed_round_a_cycle_runs.nika (probe 3, .stdout 1) and a_recursive_function_hands_a_new_lambda_to_itself.nika (the deep form: fn deep(n: i64, known: fn(ref String) -> i64? sync) -> i64 sync { if n == 0 { return known("a") ?? 0 }; let more = fn(name) { known(name) ?? alt(name) }; return deep(n - 1, more) }, main deep(2, fn(_name) { null }), .stdout 1). lower-std fixpoint clean; cargo check --profile fast -p nikaia-std --features compiler clean (without the feature tools is not checked). Commit lowering: a fn parameter handed round a call cycle is a &dyn Fn (Fixes #1023).

Done when

The three tests pass through nikaia test, tests/compiler/tests/a_trait_method_* and a_method_hands_its_callback_to_a_function.nika still pass, the guard set of .claude/agents/builder.md is green; pushed. Changelog: one line (a fn parameter handed round a call cycle is borrowed as &dyn Fn; a shadowing lambda is lent).

Stop and report if

An edit outside crates/nikaia-std/src/tools/emit_*.nika and their lowering, the checker fact files if the cycle needs one (check_facts.nika), ADR-277 and the new tests; a pausing parameter on a cycle (no probe here: report it under Found:, do not build it); an error in another agent’s file.

Not yours, never stage: every file git status shows modified or untracked at your start (among them crates/rs2nika/convert/nikaia.*, docs/selfhosting-plan.md), and every docs/briefs/* file except your report. If lowered.rs shows another hunk, stage only yours.

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/b1023-fn-param-cycle.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 is no deviation: it goes into the commit message; nor is building two steps in one commit when every step is built, nor a step another commit on develop already built: name the commit in Body:, nor a test’s expected output taken from what the program really prints: say so in Body:); how you verified goes under Tests:. A noop says why under Tests:, 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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