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agent: builder model: opus max_turns: 100 budget_usd: 20 —

Brief: #995, a ?? m.get(k) with an owned a: T? reaches rustc

Score: bug lane; blocks K3a part 2 (k3a2-comptime-compute.md, next in the batch: known(name) ?? ahead.get(name) in computed). S=1, a lowering defect on the self-hosting path. Builders have no GitHub: the issue text is copied below.

Problem

a ?? m.get(k), where a: T? is owned and m.get(k) is a map’s element, passes the checker and reaches rustc: the fallback lowers as Option<&T> beside the owned Option<T> (E0308). return m.get(k) alone as i64? runs (the lowering copies it), so only the ?? fallback position misses the copy. K3a part 2 (#125, comptime_run.rs::computed) needs exactly this form: known(name) ?? ahead.get(name) over BuildValue.

Probe (target/fast at about 0.0.1305, 2026-10-30)

use std::collections

fn first(a: i64?, m: collections::BTreeMap[String, i64], k: String) -> i64? {
    return a ?? m.get(k)
}

fn word(a: String?, m: collections::BTreeMap[String, String], k: String) -> String? {
    return a ?? m.get(k)
}

fn main() {
    let mut m: collections::BTreeMap[String, i64] = collections::BTreeMap()
    m.insert("b", 2)
    let mut w: collections::BTreeMap[String, String] = collections::BTreeMap()
    w.insert("b", "two")
    print((first(null, m, "b") ?? 0).to_string())
    print(word(null, w, "b") ?? "none")
}

rustc: Expected Option<_>, found Option<&_> at :4 (help: copied) and Expected Option<String>, found Option<&String> at :8 (help: cloned). Expected: 2, two - or, for the text and a struct, a refusal by the checker with a .clone() help if ADR-282 D7 says that copy is the program’s to write. Never rustc.

A plain return m.get(k) as String? from a lent map is a defect of its own (#996), not this one’s.

Locations

To build

  1. Re-run the probe on a fresh cargo build --profile fast -p nikaia. Passes: Result: noop, the probe as a test anyway (pins it).
  2. Check ADR-282 D7/D9 for whether the copy of a text or struct element is written by the compiler at this meet or refused; numbers are copied either way (as return m.get(k) already is). Build that in the lowering (or the checker’s refusal), for numbers, text and a struct value (BuildValue-like enum with a String part).
  3. Test (the three element kinds, also inside a lambda: let wider = fn(n) { known(n) ?? more.get(n) } over a local map, handed on as fn(String) -> T? sync); lower-std fixpoint; the it guard set. Commit Fixes #995.

Notes

Read docs/builder-facts.md; CLAUDE.md rules for builders; report form of docs/architect.md, Briefs and reports.

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/b995-coalesce-map-get.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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