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agent: builder model: opus max_turns: 80 budget_usd: 16 effort: medium —

Brief: #1127, a T? field narrowed by a null arm (#886) reaches rustc as Option inside an or-pattern

Blocks: S1 (main lane, brief not written yet: sysroot.rs facade matches Item::Fn { name: Some(name), .. } | Item::Enum { name, .. })

Score: compiler bug, checker accepts and rustc refuses (an accepted program must build). Location suspected, not traced: medium.

Facts: Where things live, Traps already met, Commands

Issue #1127

enum Item { Fn { name: String?, public: bool }, Enum { name: String, public: bool }, Other }
fn names(items: ref Vec[Item]) -> Vec[String] {
    let mut out: Vec[String] = []
    for item in items {
        let name = match item {
            Item::Fn { name: null, .. } => continue,
            Item::Fn { name, public: true } | Item::Enum { name, public: true } => name.clone(),
            else => continue,
        }
        out.push(name)
    }
    return out
}
fn main() {
    let v = [Item::Fn { name: "a", public: true }, Item::Fn { name: null, public: true }, Item::Enum { name: "e", public: true }, Item::Other]
    println(names(v).join(","))
}

Expected a,e. Got (architect, target/fast/nikaia at c60babc2) rustc E0308: &Option<String> against &String, “a binding must have the same type in all alternatives”. Without | Item::Enum { name, public: true } it prints a: #886’s rule works for a single pattern.

Locations

Suspected, not traced: the emitter’s struct-pattern writer applies #886’s Some(name) only to a whole arm that is not an or-pattern, or the checker records the narrowed position (CheckFacts::set_positions, check_facts.nika) for the arm and not per alternative. Resolve both with nikaia run scripts/where.nika -- set_positions and the callers of the struct-pattern writer in crates/nikaia-std/src/tools/emit_*.nika. emit_core_leaves.nika holds another agent’s uncommitted edit: do not stage it.

To build

  1. Each alternative of an or-pattern gets #886’s narrowing on its own: the Item::Fn alternative writes Some(name), the Item::Enum one name.
  2. Test tests/compiler/tests/a_narrowed_field_in_an_or_pattern.nika + .stdout: the probe. Fails on the unfixed binary.
  3. lower-std and its fixpoint.

Commit emit: a narrowed field in an or-pattern alternative is Some(..) (Fixes #1127).

Rules

Report (at most 15 lines, to exactly /home/user/Nikaia/docs/briefs/bugs/b30-1127-narrowed-field-or-pattern.md.report.md)

Fields of docs/architect.md, Briefs and reports: Result, Files, Commit, Body (optional), Fixes, Pushed, Tests, Gate, Deviation, Found, Needs, Changelog, Owner (a question for the owner), Disk freed, for a move Rs2nika: (the kept N of M lines of rs2nika --score and the three costliest hand edits, else none), and last Attribution: with your commit trailer lines below it.

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