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agent: builder model: opus max_turns: 80 budget_usd: 15 —

Brief: #938, a loop over an untyped list of numbers hands &i64 to a by-value call (rustc)

Score: compiler bug (false acceptance: the checker passes, rustc refuses; every for n in [1, 2]-style loop whose element reaches a call), bugs-first default of docs/priorities.md. Filed 2026-10-29, re-probed by the architect at 4fa242a6.

Issue #938 (copied, probes widened at 4fa242a6)

fn twice(n: i64) -> i64 {
    return n * 2
}

fn main() {
    let row = [1, 2]
    let mut total = 0
    for n in row {
        total += twice(n)
    }
    println(f"{total}")
}

nikaia check accepts it; nikaia run fails in rustc: Mismatched types at twice(n), “Consider dereferencing the borrow”. Lowered: for n in row.iter() { total += twice(n); }.

Probe matrix (architect, 4fa242a6): | Row | Today | | :— | :— | | let row = [1, 2], for n in row { twice(n) } | rustc mismatched types | | for r in [[1, 2], [3]] { for n in r { twice(n) } } | rustc mismatched types | | let grid = [[1, 2], [3]], nested for r in grid { for n in r ..} | rustc mismatched types | | for n in grid[0] { twice(n) } | rustc mismatched types | | let row: Vec[i64] = [1, 2], for n in row { twice(n) } | runs, 6 |

So the mechanism exists and works for a declared element type: the checker records copied_loop_bindings and the emitter writes let n = nikaia_std::num::value(n); at the body’s opening. Only the element type of an untyped list literal (and lists of them) misses it.

Locations

To build

  1. The checker records an untyped list literal’s integer element (and the inner element of a nested one, and an indexed row grid[0]) as copied_loop_bindings, as it does for Vec[i64]. Test tests/compiler/tests/a_loop_over_an_untyped_number_list_copies.nika + .stdout (6, 12, 6, one a line), the program below; run by the architect at 4fa242a6: three rustc errors (lines 9, 16, 22). Commit check: a loop over an untyped number list binds the value (#938), Fixes #938.
fn twice(n: i64) -> i64 {
    return n * 2
}

fn main() {
    let row = [1, 2]
    let mut total = 0
    for n in row {
        total += twice(n)
    }
    println(f"{total}")
    let grid = [[1, 2], [3]]
    let mut t = 0
    for r in grid {
        for n in r {
            t += twice(n)
        }
    }
    println(f"{t}")
    let mut u = 0
    for n in grid[0] {
        u += twice(n)
    }
    println(f"{u}")
}

If one row has a second cause (the fix covers the others, that row still fails), name the row and the rustc message under Found:, commit the test without that row and the fix, and say so in Body: (the architect files it).

Done when

The test passes through nikaia test and fails on the unfixed compiler; a_loop_over_copies_binds_the_value still passes; lower-std fixpoint clean; cargo build --profile fast -p nikaia; the guard set of .claude/agents/builder.md green; pushed. Changelog: one line (a loop over a list of number literals with no declared type binds each number by value, as one over Vec[i64] does). Stop about 15 turns before max_turns, commit what is finished and report partial.

Stop and report if

An edit is needed outside crates/nikaia-std/src/tools/check_*.nika, emit_stmt.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/b938-loop-copy-inferred-list.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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