t27b: return undefined in a void fn is a plain return (Closes #7402)

[gHashTag]

Refs #6063. Closes #7402.

What

Ported specs end a -> void fn with return undefined; (a deinit that frees nothing, an early exit). In Zig undefined coerced to void is the one void value, so the statement is a plain return;. t27b now lowers it that way instead of refusing it as ExprReturn. Inside a test block it stays refused: there the Zig result is an error union, and an undefined one is no value t27b can name.

Conformance first

specs/tri/t27b/conformance/return_undefined_void.t27, 4 tests: the work before the statement is kept, the work after it is skipped, in a branch, at the end of a body, and alone in a fn that is called or not. Reference: 4 pass, 0 vacuous. t27b: the same 4.

• 5 mutants of the spec: each fails the same tests in t27b and in the reference.
• 10 probes: 5 pass in both, 1 fails in both, 1 blocked in both, 1 (in a test block) an honest t27b refusal, 1 (a non-void fn returning undefined) refused by t27b where the reference passes 0 tests.

Disclosed gap

A statement after return undefined; is unreachable code, which the reference refuses to compile; t27b runs the file. t27b does not check unreachable code in general, so this gap is older than this family; it is not widened here beyond the new statement.

Numbers

• cargo test -p t27b on the lab: all suites pass.
• Lab corpus at 74b0689 vs master 05e633d (--jobs 2): mismatch 0, crash 0, timeout 2 (the same load-bound timeouts as other lane runs). t27b passes 827 reference-pass files, up from 823: gen_ring_buffer.t27, gen_rtree.t27, gen_wal.t27 (blocked -> pass) and the new conformance spec. gen_work_stealing.t27 gets past ExprReturn and now stops at codegen FnDecl(frame size) (16448 bytes of local aggregates; the limit is 16384). No file regressed. No interpreter fallback, no skipped tests.
• Ledger (docs/reports/t27b_expectations.json): the four files above added as pass, gen_work_stealing -> codegen FnDecl(frame size). Recounted: pass 533 -> 537, pass_vacuous 262, not_pass 25. No duplicate paths.

Foreign code

cli/t27b/src/lower.rs and cli/t27b/tests/source.rs are Rust. The owner approved foreign Rust in cli/t27b for #6063 and asked agents to set the owner-approved-foreign label themselves ("add the label yourself and do the foreign part", translated). Both files are listed in tools/policy/foreign-exceptions.txt.

NOW entry: docs/now/2026-10-07-t27b-return-undefined.md.

Foreign-line budget (#7371)

Master's check_budget() on git diff --numstat origin/master...HEAD: allowed (exit 0). cli/t27b/src/lower.rs +9, cli/t27b/tests/source.rs +39, total 48 of 80. The Rust test first had 45 lines; the release-only case was dropped from it because return_undefined_void.t27 already checks that shape against the reference (re-run on the lab: return_undefined_in_a_void_fn ... ok).

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