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agent: builder model: opus max_turns: 70 budget_usd: 16 —

Brief: #1027, a label on a one-element group binds its value

Score: a compiler defect found by y8 (#1026); bugs before moving. Nothing in yaml waits for it (the y2 draft labels rules directly), so it is small and first.

Issue (#1027, copied)

Found by the y8 builder (#1026, 2026-10-31). Part II 10.8: ( p ) is grouping and yields as p; x:p binds what p yields. So n:(P) binds what P yields. Today nikaia check passes and rustc refuses the emitted grammar: Cannot find value 'n' in this scope. n:(P | Q) (two alternatives) and m:(P)? work; n:(P), k:((P)), s:(text(digit+)) do not.

Probe (the architect ran it on 7f6b8151):

struct N { v: i64 }
grammar G {
	rule WS = "" -> { }
	entry rule top -> i64 = n:(P) eof -> { n.v } | "?" m:(P)? eof -> { (m ?? N { v: 9 }).v } | "(" k:((P)) eof -> { k.v } | "t" s:(text(digit+)) eof -> { s.len() as i64 }
	rule P -> N = "&" -> { N { v: 1 } }
}
fn main() {
	for t in ["&", "?", "?&", "(&", "t123", "x"] {
		let r = G::top(Bytes(t.bytes().collect())) catch {
			println("error")
			continue
		}
		println(r)
	}
}

Today: three Cannot find value (n, k, s). Expected (the same program with the parentheses removed prints it): 1 9 1 1 3 error, one per line.

Cause (architect)

The emitter writes the group as written (n:(P), emit_pattern.nika, Pattern::Group arm, line 104). The backend, winnow-grammar (rev 3b2dd3d, a pinned git dependency this repository cannot change), reads a one-alternative group with a label as a scope of its own: the name stays inside. A group around one element means nothing beyond that element (10.8: “as p”), so the emitter writes the element without the parentheses when the group holds one element that is neither a sequence nor a choice, nested groups included (((P)) is P). A group around a sequence or a choice keeps its parentheses.

Locations

To build (one commit)

  1. A group holding one element that is not a sequence or a choice is emitted as that element (recursively), so a label on it binds what the element yields. Test tests/compiler/tests/grammar_a_label_on_a_one_element_group_binds_its_value.nika with .stdout: the probe above. Run it on the unfixed binary first (it must fail), then the tests whose names start with grammar_.

Done when

The new test passes, the grammar_ tests pass, the gate of .claude/agents/builder.md (#852) is green, the commit says Fixes #1027. Changelog: one sentence (this changes crates/).

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/yaml/y9-1027-group-label.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_01UVxJDveRM5qKZKyCqq6Y24

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