BFM-decomp/cookbook/C0096.md

§85 — The RETURN-axis fleet widen is ALL-OR-NOTHING: widening the shared header alone guarantees a conflict in the source overlay (Phase 29 SESSION-20, func_8012CC88 / func_8014D12C)

§73 says a def-side self-decl conflict has two axes — RETURN (fleet widen, T2/R22) and PARAMS (casts at each use, T0). This is the RETURN axis measured end-to-end, including the way it fails.

The conflict

A remapped member's byte-true definition needs a return value the fleet declares as void:

draft def :  s32  func_8012CC88(s32, s32, s32)     <- the return keeps a local alive (§30#2)
fleet decl:  extern void func_8012CC88(s32, s32, s32)
           -> cc1: conflicting types for `func_8012CC88'

THE FAILURE MODE — widening only engine_core.h is worse than not starting

Widening the shared header banked the member in the target overlay and broke ov_SC01_077 (R22 139/140). Reason: the source overlay carries its OWN local extern void func_X(...) decls (they predate the shared macro), so a shared-header-only widen puts the two in direct conflict. The per-binary gate cannot see this — it passed on ov_SC01_000 while breaking a binary it never built. §63/§61 exactly: a T2 write set is only provable by R22.

The precondition, and how to check it in one grep

The widen is byte-neutral iff no caller consumes the return value (a call compiled against void discards it either way, so the emitted code is identical):

grep -rhoE "[A-Za-z_]\w* *= *func_XXXX\(" src/ | wc -l      # must be 0

Measured here: 0 for both functions — then the widen is safe.

Do the WHOLE axis in one edit, then R22 once

grep -rl "extern void func_XXXX(" src/ | xargs sed -i 's/extern void func_XXXX(/extern s32 func_XXXX(/g'
make clean && make extract-all && make check-all        # T2 => R22 MANDATORY

Measured scale: 3,668 decl sites across 2,688 files for two functions — and 140/140 byte-identical, tools-health green, dedup 1886/0. The site count is large because every overlay re-declares the callee locally; that is normal, not a smell.

Reading, for the next person

• A half-done axis is a guaranteed break, not a smaller win. Widen every extern void site for that symbol across src/, or widen none.
• 0 sites remaining is the completion assertion — count before and after (R32).
• The other axis (PARAMS) still uses per-use casts (§17a-1/§73); do not widen params this way.

⚠️ A verification trap that cost me a false "BYTE-IDENTICAL" report

make build BINARY=X 2>&1 | grep ... | head then echo rc=$? reports the exit status of head, not make — it is 0 even when the build failed. The output simply had no BYTE-IDENTICAL line, which is the thing to check. Assert on the expected SUCCESS STRING, never on $? after a pipe (or use set -o pipefail). A green-looking rc=0 on a failed build is the same class of self-deception R32/R35 exist to prevent — the gate was honest; my reading of it was not.