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Brief: ?? on a part of a lent tuple parameter moves it (Fixes #891)

Score: bugs lane (docs/priorities.md, bugs-first default; #891 is a checker-passes/rustc-fails bug, found by the #887 builder; probe reproduced on 2026-10-27 develop d25c5b9).

Issue text (#891)

fn f(p: (String?, i64)) -> String { p.0 ?? "none" }
fn main() { println(f(("xy", 1))); println(f((null, 2))) }

The checker passes this program. rustc refuses it: “cannot move out of p.0 which is behind a shared reference”. The parameter p is lent (a view), and ?? on its part moves the Option<String> out. The same ?? on a lent String? parameter works, and so does a struct field of a lent struct parameter (fn g(s: S) -> String { s.name ?? ".." } prints a, probed): only the tuple part misses the lowering the field gets.

Locations

emit_core_coalesce crates/nikaia-std/src/tools/emit_core_flow.nika:593 (the ?? lowering); how it recognises a lent place: compare the Expr::Field path with Expr::TupleIndex (or whatever the tuple part’s node is) in emit_core_places.nika and check_boundary.nika:261 a_place_of_a_lent_parameter / :288 place_path. Other names: nikaia run scripts/where.nika -- NAME....

To build

  1. Probes that fail on today’s develop: the program above; let x = p.0 ?? "none"; p.0 ?? "none" as a call argument; a nested (i64, (String?, i64)) part p.1.0 ?? "..". Keep the struct-field form as a control (already passes).
  2. Emit (and the checker fact, if the lowering asks it): ?? on a tuple part of a lent parameter is lowered as on a struct field of one (no move out of the view). Fix at the place where the field case is recognised, so both share one path.
  3. A test tests/compiler/tests/coalesce_on_a_lent_tuple_part.nika with .stdout, holding all forms of step 1 and the struct control. Last commit Fixes #891.

Done when

The new test; nikaia test tests/compiler --both-settings; the guard set of .claude/agents/builder.md; lower-std unchanged or changed only where a tools file used the form (say which under Tests:). Changelog: one line (a lent tuple part under ?? no longer fails in rustc).

Stop and report if

An open design choice; an error in another agent’s file.

Read docs/builder-facts.md.

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/b891-lent-tuple-part-coalesce.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. 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); 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: 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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