_file_scope_decls() hoists a region's file-scope decls into the carried layer. Its _SAFE_TYPE guard
only ever whitelisted BUILTIN base types — so a decl naming a carried file-local type was recognised
by _HOIST_RE as hoistable and then SILENTLY DROPPED.
THE COMMENT ON _SAFE_TYPE DESCRIBED A FIX THAT WAS NEVER APPLIED TO THE CODE. Verbatim: "one naming a
FILE-LOCAL type is only safe once that type is carried too — which file_scope_types() now does, so such
decls ride along after their typedef." The predicate never implemented it.
MEASURED (audit: 4,040 fleet-wide; independently re-measured here over 4 overlays: 189 drops, 32 of
them function PROTOTYPES):
* The 3,357 dropped DATA externs are LOUD — an undeclared identifier is a compile error.
* The 683 dropped function PROTOTYPES are NOT. In C89 an undeclared function is implicitly `int f()`,
so the TU still COMPILES — with the wrong return type and lost pointer-ness. And this project has
BYTE-PROVEN that the return type drives codegen (cookbook: "schedule — delay-slot fill via void
return type"; ov_SC01_077_after.c carries an `extern int`->`extern void` flip described as
byte-neutral precisely because the return type moves the delay slot).
=> A DROPPED PROTOTYPE IS A SILENT BYTE-CHANGER, armed to fire on the next carve.
Today's split is green only because the source redundantly re-declares externs per fn-group, so most
items happen to carry their own decl. That is luck, not design.
Two of the rejected base types were not even file-local: `uint` (139 drops) and `code_fn` (21) are
DEFINED IN src/shared/engine_types.h, which engine_core.h pulls into every region — the predicate was
rejecting INCLUDE-PROVIDED types it had no reason to reject. `volatile` (3) fell off because the
qualifier group had `const` but not `volatile`.
FIX: implement what the comment promised. A decl is hoistable if its base type is a builtin, OR is
carried by this layer's own file_scope_types, OR is provided by the shared headers (108 type names
parsed from engine_types.h + common.h — including FN-PTR typedefs, whose name sits inside the parens
and which every name-before-';' pattern misses; those were exactly the 5 residual drops).
COVERAGE ASSERTION (R32): a line _HOIST_RE recognises as hoistable but that cannot be placed is now a
HARD FAILURE with the base-type histogram printed, not a silent no-op. Verified safe: 0 residual drops
across 5 overlays. This one check would have surfaced all 4,040 the day the first split shipped.
No build impact (the isolator runs only when carving); --dry-run clean; tree unchanged.
Drew T
2086b15b48