BFM-decomp/cookbook/C0221.md

§200 — THE ALIAS IS THE UNIVERSAL DECLARATION ESCAPE: stop negotiating with the TU's spelling (P31 S55)

The situation this replaces. A byte-verified draft is refused because its declaration of a symbol disagrees with the destination TU's, or with a slate-mate's. §183's playbook answers this by NEGOTIATING — adopt the TU's spelling and narrow at the use site, cast at the call, view-cast the pointer — and that works 18 times in 20. The other 2 are §183.3's hard limit: when the TU's spelling forces an &D_x your spelling does not, every cast escape loses the match from instruction #1, and the entry says to report IMMOVABLE with a TU edit.

There is a third option, and it always works: don't share the C identifier at all. The link name is the only thing that has to agree. gcc's asm-label binding (§37/§124) lets a draft declare its OWN identifier, with its OWN type, bound to the same symbol:

extern u8  aD8018A800[] __asm__("D_8018A800");   /* TU spells it `extern s32 D_8018A800;`     */
extern void *aD801EF9FC  __asm__("D_801EF9FC");  /* TU spells it `extern s32 D_801EF9FC;`     */
s32 aF80185B48(s32 a0)   __asm__("func_80185B48");  /* TU prototypes it `void func_…(s32);`   */

Same symbol, same relocation, same bytes — and nothing left for the slate to disagree about. It works on the DEF side too, which matters because a definition's own signature is the one place a cast cannot reach (§183.3, §20's DEF-side wall): give the definition a private C name and bind it.

Measured, wave Y's recovery lane (P31 S55). Five gate drops, five different refusal classes: array-vs-scalar where reconcile_slate's automatic array fix had broken the match; a slate-mate's private struct type; a void * vs s32 global; and two DEF-side return conflicts, one of them a function whose only in-TU use TAKES ITS ADDRESS as a callback. All five aliased, all five still MATCH, all four re-gated banked (the fifth was already banked by then). Total agent cost: zero — the API was down with 529s and the whole lane was done by hand in three edits.

WHEN NOT TO REACH FOR IT. This is the escape hatch, not the first move, and the ordering matters:

1. Adopt the TU's spelling if it costs no bytes — that is what wave law 2 is for, it keeps the file readable, and it is right 18 times in 20.
2. Cast at the use site (§183's playbook) when the types differ but the access does not.
3. Alias when 1 and 2 cost bytes, or when the conflict is DEF-side and has no cast form. An alias is a small readability debt: a reader sees aD8018A800 and must follow the label to learn it is D_8018A800. Pay it deliberately, with the comment explaining WHICH spelling you could not use and why — every alias in the tree carries one.

The tooling had to learn this idiom the same day. sym_of was returning __asm__ as the symbol for every aliased declaration (an identifier followed by (, matched before the real one), so aliased declarations all collided under that name — 1 byte-verified draft dropped and 2 phantom CONFLICTING-EXTERNs on the very slate this lane was recovering. Fixed with a 0-regression control over 1,210 changed verdicts (899 of them one symbol: the idiom is fleet-wide). A project idiom the tools cannot parse is an idiom that silently costs work — third instance of the §192 class.

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