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Brief: #1046, an untyped let lambda handed to a fn parameter that may pause lowers as a plain closure

Score: compiler bug (b1043’s Found, row d), false acceptance (nikaia check passes, rustc refuses); bugs-first default of docs/priorities.md. Small, one cause traced.

Issue #1046 (copied)

Found by b1043 (row d), probed by the architect on the fast binary after 569bb244. #997 is incomplete for a callee whose fn parameter may pause.

Problem

fn apply(f: fn(i64) -> i64) -> i64 {
    return f(1)
}
fn main() {
    let add = fn(x) { x + 3 }
    println(f"{apply(add)}")
}

Expected 4. nikaia check passes; rustc refuses: “i64 is not a future” (required by a bound in apply). Lowered:

async fn apply(f: &impl AsyncFn(i64) -> i64) -> i64 { f(1).await }
let add = |x| { x + 3 };
println!("{}", apply(&add).await);

Rows:

Cause (traced)

check_core_stmt_let (check_core_stmt.nika:544-564) types the lambda by its first use (#997: lambda_params_by_use, run_code) but does not record that the use may pause. An inline lambda gets that from check_core_args_lambda (check_core_args.nika:101-115: future_lambdas, run_lambdas when !is_sync), and emit_arg_future (emit_core_args.nika:600) writes async |..|. The let’s lambda is emitted by emit_core_closure (emit_core_flow.nika:235-280), which writes async only for pausing_lambdas (std entries, check_crossings.nika:432), so it comes out a plain closure.

Locations

To build

  1. A lambda bound by an untyped let and typed by a use whose fn type may pause (no sync) lowers as the language below’s async closure, as the same lambda written into that call does: the checker records it (a fact keyed by the closure’s node_key; run_lambdas or a new one, say which in Body:), and emit_core_closure writes async for it. Rows a, b run; c, d, e unchanged. A later direct call of the name (add(1)) is #833’s, not this issue’s.
  2. Test tests/compiler/tests/an_untyped_let_lambda_run_by_a_pausing_parameter.nika + .stdout with rows a, b and c. Commit emit: an untyped let lambda run by a parameter that may pause is an async closure (Fixes #1046).

Done when

The new test passes through nikaia test --project tests/compiler and fails on the unfixed compiler (run its rows on the current binary before your first edit and keep the outputs under Tests:); the tests nikaia run scripts/where.nika -- check_core_stmt_let emit_core_closure check_core_args_lambda lists pass; tests/compiler/tests/let_lambda_string_param.nika, a_let_lambda_handed_where_a_function_is_kept.nika and an_untyped_let_lambda_in_a_fn_field.nika 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. Stop about 15 turns before max_turns, commit what is finished and report partial.

Stop and report if

An edit is needed outside check_core_stmt.nika, check_core_args.nika, check_facts.nika, emit_core_flow.nika, emit_core_args.nika, their lowered twins (and lowered.rs), files lower-std rewrites because of your change, and the new test; an error in another agent’s file; the fix turns out to need a rule Part I or #997 does not give (then Result: blocked, Needs: owner decision). tests/compiler failures from a binary vanishing mid-run (another agent rebuilding) are not yours: run the named tests singly instead.

Not yours, never stage: every file git status shows modified or untracked at your start (today among them crates/rs2nika/convert/*, a stray file whose name starts ork in the tree)), and every docs/briefs/* file 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/b1046-let-lambda-pausing-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 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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