Result: done
Files: crates/nikaia-std/src/tools/check_core_args.nika crates/nikaia-std/src/tools/lowered/check_core_args.rs tests/compiler/tests/flat_map_seq_result.nika tests/compiler/tests/flat_map_seq_result.stdout docs/briefs/bugs/b21-1097-flat-map-seq-result.md.report.md
Commit: check: flat_map binds its element from a Seq, Vec or Array result (Fixes #1097)
Body: The rule sits in check_core_args.nika check_core_args_lambda, beside #1078’s ?? rule: where the written result is a Vec[U] the receiver left unbound (Vec[?]) and the body comes to a Seq[X], Vec[X] or Array[X] with X known, the lambda reports Vec[X] (check_core_args_walked), so the plain bind gives U = X and the argument still fits Vec[$U]. A first try in ty.nika bind (a Vec[U] result pattern binding the element of a Seq) could not stand: reporting the body’s Seq makes the argument check (check_arguments.nika, outside the brief’s files) refuse it with NK1102, and making a Seq fit a Vec in Ty::fits would be the subtyping ADR-013 D7 refuses. So nothing is accepted that the old checker refused, only U is filled in. Other walkables are left as before: the BTreeSet lambda in ledger_items.nika:324 still reports Vec[?]. flat_map needed no mapped rule in text_tiers.nika: the trim() row lowers .map(|p| { p.trim() }) with the parts kept as views. The test’s named-function row runs over split, because a named function over iter() fails in rustc (fn(&&_)) on the unfixed binary too, with map as well (#1098’s family).
Fixes: #1097
Tests: new flat_map_seq_result: unfixed binary FAILED (NK1245 twice, the Seq rows), fixed passes with –both-settings; probes on the unfixed binary: Seq row NK1245, Vec row printed 4, named row over iter() a rustc fn(&&_) mismatch (same with map, #1098), over split printed 4; filter_map_split_parts passes; it nullable seq_reductions views_kept map_trust templates 126 passed; grammar_bytes result_source modules package_graph build_time_dependencies 119 passed; lower-std fixpoint clean (the second lower-std changed no lowered file); cargo fmt, clippy clean
Gate: green
Deviation: none
Found: none
Needs: none
Changelog: The function flat_map takes may hand back a Seq as well as a Vec or an Array, and the next lambda gets its element’s type (for example the parts of a split, still as views) (#1097).
Owner: none
Disk freed: 0.05 GB (scratch binaries); project-tests cache 2.6G, disk 3.5G free of 252G; new cache entries over 100 MB from the it runs: build-time-parsers 1025 MB, build-time-bundle 546 MB, run-bundle 288 MB, two rlib 272 and 267 MB
Rs2nika: none
Attribution:
Co-Authored-By: Claude Opus 5.5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01GFeMqxRbtUMiGVyDMxsvEq
Pushed: local dcc8d12e3a3e9c88c3209d3fa3e7fadbd6cc4975
