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Brief: #1029, a pausing kept lambda typed -> i64 hands back i32 literals (rustc)

Score: compiler bug (false acceptance: the checker passes, rustc refuses; every typed let lambda with a literal result), bugs-first default of docs/priorities.md. Found by b1017.

Issue #1029 (copied)

Found by b1017 (report of ee9aa6a6), widened by the architect’s probes at 61df8cf5.

Problem

A pausing lambda kept under a declared fn type whose result is an integer type loses that type: every bare integer literal it hands back (tail, arm, branch, return) is i32 in rustc, and rustc refuses.

fn main() {
    let k: fn(i64) -> i64 = fn(x) { 5 }
    println(k(1))
}

Today: Expected {async block ...} to be a future that resolves to i64, but it resolves to i32. Expected 5.

Lowered: Kept(Arc::new(move |x| Box::pin(async move { 5 }) as nikaia_std::func::BoxedIn<'_, _>)). The unsizing coercion to dyn Fn does not carry the expected type back into the closure, and _ leaves the literal to rustc’s default i32.

Probe matrix (all at 61df8cf5): | Row | Today | | :— | :— | | let k: fn(i64) -> i64 = fn(x) { 5 } | i32 error | | ... = fn(x) { match x { 1 => 5, else => 7 } } | i32 error | | ... = fn(x) { if x == 1 { 5 } else { 7 } } | i32 error | | ... = fn(x) { if x > 9 { return 3 } 8 } (two lines) | i32 error | | let m: fn(i64) -> i64? = fn(x) { if x > 0 { 4 } else { null } } | Option<i32> error | | let k: fn(i64) -> u8 = fn(x) { 5 } | i32 error | | let k: fn(i64) -> i64 = fn(x) { x + 5 } | 6 (fine: x fixes it) | | let k: fn(i64) -> f64 = fn(x) { 5.0 } | 5 (fine: f64 is rustc’s default) | | let k: fn(i64) -> i64 sync = fn(x) { match x {..} } | 5 (fine: not boxed) | | apply(fn(x) { match x {..} }), apply(f: fn(i64) -> i64) | 5 (fine: AsyncFn bound) |

Locations

To build

  1. The checker records a pausing kept lambda’s declared result where it is an integer type or an optional of one (a new fact beside kept_lambdas, holding the Ty, or the spelling); the emitter writes that spelling in place of the cast’s _ (BoxedIn<'_, i64>, BoxedIn<'_, Option<i64>>), and keeps _ everywhere else. A lambda that may fail (throws) keeps _: not probed, out of this step. Test tests/compiler/tests/a_kept_lambda_keeps_its_integer_result.nika + .stdout, rows 1-6 of the matrix in one program, printing 5, 5, 7, 8, 4, 5 (calls k1(1), k2(1), k3(2), k4(1), m(1) ?? 0, k6(1)). Commit emit: a kept lambda's integer result types its literals (#1029), Fixes #1029.

Rows 1-6 were each traced by the architect to the _ of the cast (the generated .rs read for row 1; rows 2-6 give the same rustc message at the same column). The b1017 builder tried state.number_asked on the lambda tail; it did not help (and could not reach a return), so the fix is the cast.

Done when

The test passes through nikaia test; 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 under a declared fn type with an integer result hands back that type’s literals).

Stop and report if

An edit is needed outside crates/nikaia-std/src/tools/emit_*.nika, check_*.nika, check_facts.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.

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/b1029-kept-lambda-int-result.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:, 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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