agent: builder model: opus max_turns: 80 budget_usd: 15 —

Brief: a `catch` on a call used as a statement drops the call’s value (#885)

Score: bugs lane first (docs/priorities.md; CLAUDE.md Bugs before moving): nikaia check passes a program rustc refuses, found by rs2nika lane H (#877), on the code #884 (0.0.1239) just touched.

Issue #885 (copied)

Probe (half needs its statements on separate lines):

enum Refused { Negative }
impl Error for Refused { fn message(ref self) -> String { return "negative" } }
fn half(n: i64) -> i64 throws {
    if n < 0 { throw Refused::Negative }
    return n / 2
}
fn main() {
    let mut failed = false
    half(-4) catch { failed = true }
    half(4) catch {}
    print(failed)
}

Seen: nikaia check passes, and rustc refuses the lowered code: “match arms have incompatible types”. The Ok arm is the i64, the Err arm is (). Expected (decided, architect 2026-10-27): a call used as a statement drops its value, so its catch block need not hand one back. Prints true. Not NK1103: a statement has no value to match.

Locations

Emitter: crates/nikaia-std/src/tools/emit_core_flow.nika:829 emit_core_try_catch; the Ok arm is written at :944-951 (Ok(value) => value, and the #884 forms Some(value) / value.to_owned() chosen by self.facts.nullable_catches / owned_catches). Called from emit_expr.nika:381.
Checker: crates/nikaia-std/src/tools/check_core_flow.nika:1004 check_core_flow_try_catch; the #884 pattern records a fact at :1061 (state.facts.nullable_catches.insert(node_key(guarded_expr))); the fact sets live in check_facts.nika:341 / :538.
Statement position: Stmt::Expr arms in emit_below.nika, emit_core_below.nika, emit_core_reads.nika; nikaia run scripts/where.nika -- NAME... for the rest.
Tests beside: tests/compiler/tests/a_catch_that_hands_back_null_widens_to_nullable.nika (+.stdout), a_catch_that_hands_back_another_type_is_refused.nika (+.stderr).

To build

A catch whose guarded expression is a whole Stmt::Expr lowers with its success value dropped (Ok(_) => {}) and its handler’s value dropped too, whatever the block hands back. How the emitter learns the position (a fact the checker records like nullable_catches, or the statement emitter telling emit_core_try_catch through its flow) is your choice: say which in the commit message. A catch in a let, an argument, a return or a block’s last expression that gives the block’s value is unchanged (the existing catch tests show it).
Test tests/compiler/tests/a_catch_on_a_call_statement_drops_its_value.nika + .stdout: the probe, plus a statement catch whose block hands back a value (half(1) catch { 0 }), and one inside a loop body. Run it against the unfixed compiler first (say so under Tests:).

Done when

lower-std rerun (the lowered files of the touched .nika change, nothing else); nikaia test --both-settings --project tests/compiler -- catch and the guard set of .claude/agents/builder.md; it filter catch. Commit: a catch on a call used as a statement drops the call's value (Fixes #885). Changelog: one line (a catch on a call used as a statement no longer needs its block to hand back the call’s type).

Stop and report if

A statement catch turns out to be refused or typed elsewhere by the specification text (Part I 7.1) in a way that contradicts the decision above: report blocked with the quote.

Rules (CLAUDE.md; this block is pasted into every brief, also a fix-forward)

Read docs/builder-facts.md first. develop in the main checkout; git status at start: other agents’ uncommitted files are not yours, never touch or stage them.
git add <files> only; commit after each step; git push origin develop fast-forward, no rebase (rejected: report Pushed: rejected, go on committing).
Before pushing: cargo fmt, cargo clippy --all-targets -- -D warnings, scripts/pre-push-check.sh, and the gate of .claude/agents/builder.md (#852).
No version lines, CHANGELOG.md, badges; no new NK code. New tests in Nikaia only; never the full suite. No foreground sleep, no polling; scripts in files.
Disk: NIKAIA_CACHE_LIMIT=2G; at most two clones with target/ in the machine; delete your own clone’s target/ and target/release/nikaia-scratch at the end; report GB freed.

Report (at most 15 lines, to exactly `/home/user/Nikaia/docs/briefs/b885-catch-statement.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, 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); 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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Brief: a catch on a call used as a statement drops the call’s value (#885)