2.3.12 Forward declarations

When the compiler encounters a forward declaration of a struct, union, or enum, it emits a type of kind CTF_K_FORWARD. If it later encounters a non- forward declaration of the same thing, it marks the forward as non-root-visible: before link time, therefore, non-root-visible forwards indicate that a non-forward is coming.

After link time, forwards are fused with their corresponding non-forwards by the deduplicator where possible. They are kept if there is no non-forward definition (maybe it’s not visible from any TU at all) or if multiple conflicting structures with the same name might match it. Otherwise, all other forwards are converted to structures, unions, or enums as appropriate, even across TUs if only one structure could correspond to the forward (after all, all types across all TUs land in the same dictionary unless they conflict, so promoting forwards to their concrete type seems most helpful).

A forward has a rather strange representation: it is encoded with a ctf_stype_t but the ctt_type is populated not with a type (if it’s a forward, we don’t have an underlying type yet: if we did, we’d have promoted it and this wouldn’t be a forward any more) but with the kind of the forward. This means that we can distinguish forwards to structs, enums and unions reliably and ensure they land in the appropriate namespace even before the actual struct, union or enum is found.