cg_structlit_fill/cgstructlitfill dispatched scalar field stores as {1->MOVB, 4->MOVL, else->MOVQ} with no fsz==2 case, so a 2-byte field was stored with an 8-byte MOVQ. Interior over-stores were harmlessly overwritten by the next field, but the LAST field at the frame edge corrupted the saved base pointer: an (S|e) union success variant places the struct payload after the 8B tag, landing the last field at -4(BP), so MOVQ AX,-4(BP) wrote into saved [BP] and POPQ BP restored garbage — a silent both-stage caller-frame clobber. Route the scalar store through the existing fldstoreop/fieldstoreop helper ({1->MOVB,2->MOVW,4->MOVL,else->MOVQ}), both stages; the #13 graduation comments already pre-documented this resolution. Pure width fix, no loud-stop (scalar widths are always {1,2,4,8} and narrowing is always correct). Value-asserting pin: an i64 sentinel live across the union-maker call (detects the clobber directly) + all members, with a non-union control.
Hojun-Cho
8fa59f9d45