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Brief: #1097, flat_map with a function that hands back a Seq leaves the element untyped

Score: compiler bug, a wrong refusal (NK1245) of a plainly meant form. Found by b18 (#1078, 46824f2b), which built the U? arm of bind this one sits next to. One function, cheap and certain.

Facts: Where things live, Traps already met, Commands

Issue #1097 (probe checked by the architect on target/fast/nikaia at 0510bf6f)

fn main() {
    let lines = ["a;b", "c"]
    let parts = lines.iter().flat_map(fn(l) { l.split(";") }).map(fn(p) { p.len() }).collect()
    print(parts.len())
}

Expected 3. Got NK1245 on p.

Cause (by reading, sure): crates/nikaia-std/std.contracts [fn."Seq::flat_map"] declares f: fn(T) -> Vec[U]. In crates/nikaia-std/src/tools/ty.nika, bind sends the Vec[U] pattern to bind_named, which binds only against a Ty::Named “Vec”. The lambda hands back Seq[ref String] (str::split), so U stays unbound.

Decision (architect, on the issue): flat_map’s function may hand back anything a for walks, a Vec, an Array or a Seq, and U is its element. Rust’s flat_map takes IntoIterator, so the lowering does not change.

To build

  1. Reproduce the probe on the unfixed binary, plus a Vec result row (fn(l) { [l.len(), 1] }) and a named-function row. Keep the results under Tests:.
  2. Bind U from a Seq[X], Vec[X] or Array[X] result. Where the rule goes is your choice: say which in Body:. It can sit in bind/bind_named as an element binding of a sequence-like pattern against Ty::Seq, or the contract can say Seq[U] with a Vec result fitting it. Do not widen the binding beyond the element of a walkable sequence. The text kinds of the parts must survive into the next lambda, as #1078 made them for filter_map (text_tiers.nika, ADR-293 D26). If flat_map needs the mapped rule too, give it, and say so in Body:.
  3. lower-std and its fixpoint.
  4. Test tests/compiler/tests/flat_map_seq_result.nika + .stdout with the three rows, failing on the unfixed binary. Commit check: flat_map binds its element from a Seq, Vec or Array result (Fixes #1097).

Done when

The new test fails on the unfixed compiler and passes through nikaia test --project tests/compiler --both-settings, along with the it modules that mention flat_map/filter_map (b18’s list: nullable seq_reductions views_kept map_trust templates). Also: lower-std fixpoint clean; build, fmt, clippy, pre-push-check.sh; committed locally. Changelog: one line. Stop about 15 turns before max_turns: commit what is finished and report partial.

Stop and report if

An edit is needed outside ty.nika, check_core_args.nika, check_core_method.nika, text_tiers.nika, std.contracts, their lowered twins and the new test. Or you hit an error in another agent’s file. #1098 (named fn and filter_map over iter() of copy elements) is a separate issue: do not fix it here.

Not yours, never stage: every file git status shows modified or untracked at your start, and every docs/briefs/* file except your report.

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

Report (at most 15 lines, to exactly <full brief path>.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 ...).

Report path: /home/user/Nikaia/docs/briefs/bugs/b21-1097-flat-map-seq-result.md.report.md

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