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Brief: #1120, a function type with a function parameter lowers to nested impl Trait (rustc E0666)

Score: compiler bug, a program nikaia check accepts fails nikaia run; blocks a self-hosting move. Cause traced to the function, not run: small.

Facts: Where things live, Traps already met, Commands

Issue #1120

fn twice(scoped: fn(i64, fn() -> i64 sync) -> i64 sync) -> i64 sync { return scoped(1, fn() { return 2 }) + 1 }
fn main() { println(f"{twice(fn(at, run) { return at + run() })}") }

Expected 4. Got from rustc: Nested `impl Trait` is not allowed and `impl Trait` is not allowed in the parameters of `Fn` trait bounds.

To build

  1. Reproduce (Tests:).
  2. Fix in emit_core_ty_named, crates/nikaia-std/src/tools/emit_core_types.nika (the “A parameter that is code” branch, about :150-270). The code’s own parameter types (ty.generics, emitted into params, called and elided) and its result are emitted with walk.in_parameter still set, so an inner code type becomes impl Fn too. Emit them with walk.in_parameter = false (restored after), as the optional-code branch just above does, so an inner code type becomes nikaia_std::func::Kept<dyn Fn() -> i64>; the outer parameter keeps impl Fn. The same in the for<'call> branch. Owner, 2026-11-01: the simple Kept (an Arc) form now; a form without the allocation is a later issue, filed by the supervisor.
  3. The argument side: the lambda handed to scoped must become Kept(Arc::new(..)) (the existing wrapping, emit_expr.nika :512, emit_core_flow.nika :378). If it is not, fix it where an argument meets a Kept parameter (emit_core_args.nika).
  4. lower-std and its fixpoint.
  5. Test tests/compiler/tests/a_function_type_takes_a_function.nika + .stdout: the probe; an inner function that captures a local; an inner function with a parameter and a throws result if the language allows it there. Fails on the unfixed binary.

Commit emit: a function type's function parameter is kept code, not a nested impl Fn (Fixes #1120).

Done when

The new test fails on the unfixed compiler and passes through nikaia test --project tests/compiler --both-settings; the tests named fn_|code|lambda|closure|kept pass; 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 the emitter files (emit_*.nika), their lowered twins, lowered.rs, check_*.nika where the checker’s type of the inner lambda is wrong, and the new test. Or you hit an error in another agent’s file.

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 /home/user/Nikaia/docs/briefs/bugs/b28-1120-fn-type-fn-param.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 or red <check>: your cheap checks, never none, which cuts the batch, j4 2026-10-31), 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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