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Brief: #1008, a trait method’s fn parameter (impl type and the kept callback’s clone)

Score: bug lane, a false acceptance (checker passes, rustc refuses; S=2) that blocks K3a part 2 (comptime_computed hands its callbacks to ComptimeComputeHost at every step); bootstrap week: a blocker of a move goes first. Builders have no GitHub: the issue text is copied below; every probe row was run by the architect on 3b845e0b, except the recursive trait row of step 1, which cannot run before the fix (its free-function twin prints 9): if it fails for a reason other than defects 1-2, that is a Found:.

Problem

A body-less trait method keeps its fn parameters (ADR-277 D11: run is the absence of keeps), so the parameter lowers as nikaia_std::func::Kept<dyn Fn..>. Two defects follow, both accepted by the checker and refused by rustc:

  1. The impl’s method lowers its fn parameter as impl Fn (its body only runs it), so it does not match the trait: rustc E0049 “Method ask has 1 type parameter but its trait declaration has 0”.
  2. A Kept parameter handed on is moved, never cloned: handed twice (or into a loop, or into a recursive call), rustc “use of moved value”. Kept is Clone (an Arc).

Probe (nikaia run, 3b845e0b)

trait H {
    fn ask(ref self, known: fn(ref String) -> i64? sync) -> i64? sync
}
struct Plain { base: i64 }
impl H for Plain {
    fn ask(ref self, known: fn(ref String) -> i64? sync) -> i64? sync {
        return known("a")
    }
}
fn twice[T: H](host: ref T, known: fn(ref String) -> i64? sync) -> i64 {
    let a = host.ask(known) ?? 0
    let b = host.ask(known) ?? 0
    return a + b
}
fn main() {
    let p = Plain { base: 1 }
    println(twice(p, fn(name) { if name == "a" { 3 } else { null } }))
}

Today: E0049 at the impl’s fn ask (defect 1) and “Use of moved value: known” at line 12 (defect 2). Expected: 6.

Locations

To build

  1. The impl’s method of a trait takes the trait method’s keeps for its fn parameters; a Kept handed to a Kept parameter is cloned where it is read again (any lowering rustc accepts). Test tests/compiler/tests/a_trait_method_callback_is_handed_on.nika + .stdout: the probe (6), the direct p.ask(known) row with a typed let-bound lambda (3), and a recursive row fn down[T: H](host: ref T, n: i64, known: ..) -> i64 handing known to host.ask and to itself (down(p, 3, known) prints 9). lower-std fixpoint clean (only files whose lowering changes). Fixes #1008.

Done when

The test passes through nikaia test (copy it to the scratchpad to nikaia run it alone: a file inside tests/compiler is not the project’s entry), the control rows (free functions) still print 6 and 9, the lower-std fixpoint is clean, the gate of .claude/agents/builder.md is green; pushed. Commit subject: emit: a trait method's fn parameter is kept in the impl too, and handed on by clone (#1008), Fixes #1008. Changelog: one line (programs handing a callback through a trait method now run).

Stop and report if

The checker, not the emitter, decides the impl’s type (then Found: with where); an edit outside tools/emit_*.nika, their lowering, lowered.rs’s index lines and the test; an error in another agent’s file.

Not yours, never stage: crates/rs2nika/convert/nikaia.{contracts,derived,proofs}, docs/selfhosting-plan.md, and every docs/briefs/* file that git status shows modified or untracked except your report.

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/b1008-trait-fn-param.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 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:); 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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