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Brief: .to_string() of an inferred number local or tuple field, returned as a view (#866)

Score: bug lane 1, the third of a chain (#864, #865) in the same walk; a rustc failure on a plain program. Compiler change: the supervisor raises the version.

Issue #866 (copied)

Found by the #865 builder (635b59f, report docs/briefs/b865-to-string-local-view.md.report.md). Reproduced on develop 79a95b3.

Probe.

fn f() -> String {
    let p = (3, 4)
    let s = p.0.to_string()
    return s.trim()
}
fn main() { println(f()) }

nikaia run gives error: Cannot return value referencing local variable s (rustc). With let s = (3).to_string() or t.len().to_string() it runs (#865).

Why. formats / formats_a_number in crates/nikaia-std/src/tools/buffers.nika (about lines 535-587) decide without types whether to_string makes a buffer of its own: a number literal, arithmetic, len, a call declaring a number result, or a local whose written type is not text (formats_local). A local with no type written (let p = (3, 4)) has ty: null, so formats_local says false, and a field p.0 falls to else => false.

Scope. Inferred locals bound to a number expression, and tuple fields of a local bound to a tuple literal. A struct field, a method result, a generic call stay out: they need the checker’s types (#851’s area), not this walk.

Locations. bind_let around buffers.nika:400-421 (where a_local(..) is built; Local struct line 98, a_local line 188); formats_local line 582; formats_a_number line 547. Test beside tests/compiler/tests/a_view_of_a_formatted_local_is_returned_owned.nika.

To build.

  1. Local gets number: bool (or the tuple’s per-element flags): set in the let binding when no type is written and formats_a_number(value) holds; for a tuple literal, which elements are numbers. formats_local answers true for such a local.
  2. formats_a_number handles Expr::Field { base: Variable, name } where the name is a tuple index and that element of the local was a number.
  3. Test tests/compiler/tests/a_view_of_an_inferred_number_local_is_returned_owned.nika (+ .stdout): let n = 3 then n.to_string(); let p = (3, 4) then p.0.to_string(); let q = ("a", 4) with q.1 (number) — each returned through .trim(). Must fail before the fix. lower-std, rebuild, commit Fixes #866 with the regenerated lowered/buffers.rs.

Cases that already pass before the fix stay in the test as guards; say which under Tests, not under Deviation.

Files you may change

crates/nikaia-std/src/tools/buffers.nika (+ lowered/buffers.rs), the new test. Anything else: stop and report under Needs. Known facts: 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/b866-inferred-number-local.md.report.md)

Fields of docs/architect.md, Briefs and reports: Result: done|noop|partial|blocked (the bare word first; Owner: is a question for the owner or none, never a role), Files, Commit, 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. 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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