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Brief: #1035, return h.f of a fn field out of a ref parameter is refused NK1131

Score: compiler bug (false refusal of a form Part I allows; let g = h.f in the same place runs), bugs-first default of docs/priorities.md. Found by b1031, probed by the architect at 39f8dbf6.

Issue #1035 (copied)

struct H {
    f: fn(i64) -> i64
}

fn inc(x: i64) -> i64 {
    return x + 1
}

fn pick(h: ref H) -> fn(i64) -> i64 {
    return h.f
}

fn main() {
    let h = H { f: inc }
    let p = pick(h)
    println(f"{p(1)}")
}

Refused NK1131 (“You can’t give away h.f here: h is only borrowed”). Part I 5.4 (A function type stands wherever a type does): a copy of a function value shares the one closure, so reading it out of a loan takes nothing away. let g = h.f over h: ref H already runs, and return h.n (i64) runs. Since #1031 (637bec45) the emitter clones a field read of fn type (facts.kept_fn_copies, check_core_expr.nika:316), so h.f.clone() should come out.

Cause traced: check_core_a_lent_place_given_away (check_core_leaves.nika:74) returns early for copies_plainly(ty) (check_types.nika:119: numbers, bool, char, scalar, views). The !handed_back path (line ~163) also returns for state.copied(world, held); the handed-back path (return, entered from check_core_a_field_of_a_lent_place_handed_back, line 56) asks nothing more, so a Ty::Fn is refused there.

To build

  1. A place of fn type out of a lent root is no give-away in check_core_a_lent_place_given_away (Part I 5.4): return early for a Ty::Fn beside copies_plainly (do not widen copies_plainly itself, it has 17 other callers). Test tests/compiler/tests/a_fn_field_handed_back_from_a_loan.nika + .stdout (4, 2, 3, one a line), the program below; run by the architect at 39f8dbf6: NK1131 at line 15. Check the lowered pick (nikaia lower) clones the field; if it does not, that is rustc’s message under Found:. Commit check: a fn field handed back out of a loan is a copy (#1035), Fixes #1035.
struct H {
    n: i64,
    f: fn(i64) -> i64
}

fn inc(x: i64) -> i64 {
    return x + 1
}

fn num(h: ref H) -> i64 {
    return h.n
}

fn pick(h: ref H) -> fn(i64) -> i64 {
    return h.f
}

fn main() {
    let h = H { n: 4, f: inc }
    println(f"{num(h)}")
    let p = pick(h)
    println(f"{p(1)}")
    println(f"{h.f(2)}")
}

Traced: the return h.f row. Untraced, not this brief’s: a fn element handed back out of a lent list (return hs[0]), which takes the element path.

Done when

The test passes through nikaia test and fails on the unfixed compiler; the NK1131 tests (nikaia run scripts/where.nika -- NK1131 lists them; the .nika ones under tests/compiler) 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 (a function value handed back out of a borrowed struct is a copy that shares its closure, no longer refused NK1131). 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_core_leaves.nika, check_types.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/b1035-fn-field-from-loan.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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