const RESERVED_SUBGROUP_ID_MODE: u8 = 3;Expand description
The fourth value of the two-bit subgroup-ID mode, which no draft assigns.
Drafts 16 through 20 reserve it by name and list the type bytes that carry it; draft-15 arrives at the same eight bytes by leaving them out of its table. Either way no header the decoder returns holds this value.
The codec stores a placeholder zero for both mode 1 (subgroup ID is the
first object’s ID) and this one, which is why CapCtx::subgroup_id_mode
exists at all: without it a reserved-mode header is indistinguishable from a
first-object-mode header and the elide guard reports
Refusal::WouldRedefineSubgroupId for something that is not a subgroup ID
question.