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Brief: #939, a match guard or an if/while head refuses a shift and a bit operation

Score: S=3 (a wrong refusal of plain syntax; met in a converter draft, k1 n.magnitude == 1 << 63), O=0.3 M, C=2 M (one grammar level in parser/mod.rs), area bugs. Cheap and certain.

Issue #939 (copied; widened by the architect at c30b7bc0)

fn main() {
    let m: i64 = 8
    let s = match true {
        true if m == 1 << 3 => "eight",
        else => "other",
    }
    println(s)
}

nikaia check refuses: Expected => here, but found <. (4:24). Written m == (1 << 3) it parses; let a = m == 1 << 3 outside a head parses.

Architect’s probes at c30b7bc0, if <head> { println("yes") } else { println("no") } with let m: i64 = 8:

Head Today Expected
m >> 1 == 4 parse error Didn’t expect > here. yes
m & 8 == 8 parse error Expected { here, but found &. NK1197 as in a let
m \| 1 == 9, m ^ 1 == 9 the same parse error NK1197
(m << 1) == 16 yes yes

In a let, let b = m & 8 == 8 gives error[NK1197]: Put parentheses around a bit operation next to a comparison.; so the head must give the same.

Cause (traced)

crates/nikaia/src/parser/mod.rs: the full expression chain is cmp_expr (2853) -> bit_or_expr (2872) -> bit_xor_expr (2881) -> bit_and_expr (2890) -> shift_expr (2899) -> add_expr (2921). The head chain used by match_guard (3694, KW_IF e:head_expr) and by the if/while heads is head_expr (3335) -> … -> head_cmp_bare (3442: head:head_add tail:cmp_head_tail?) -> head_add (3449): it skips the four bit/shift levels. The head chain exists to refuse a struct literal’s brace (head_primary), so the new levels must call the head rules beneath them, never add_expr.

To build

  1. Add head_bit_or, head_bit_xor, head_bit_and, head_shift between head_cmp and head_add, mirroring bit_or_expr…shift_expr (same operators, same not(..) guards, fold_binary), so a guard, an if and a while head read shifts and bit operations, and NK1197 fires there as in a let (find where NK1197 is raised; it should need no change if it looks at the tree).
  2. Test tests/compiler/tests/a_shift_in_a_guard_or_a_head.nika with .stdout: the issue’s probe (eight), a >> guard (true if m == 16 >> 1 =>), if m >> 1 == 4, and while with a shift in its head. A refusal test tests/compiler/tests/a_bit_operation_in_a_head_wants_parentheses.nika with its .stderr (NK1197 for if m & 8 == 8 {), taken from the real run. Run both against the unfixed compiler first.

Only crates/nikaia/src/parser/mod.rs and the new tests are yours. Run the parser’s tests that touch heads (cargo test -p nikaia parser style filters, and nikaia test filters on match_guard, guard, if_), never the full suite.

The commit that finishes it says Fixes #939.

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/bugs/b2-939-shift-in-heads.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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