FunctionCall

An invocation of a catalog function, or of the @index system function.

This is the one shape for both: common_types.json gives IndexSystemFunction its own definition, but it differs from a catalog call only in that call is the constant @index and that it takes no catalogId — see isSystemFunction. FunctionCommon likewise collapses into the catalogId property here.

args is null when the wire object carried no args key at all, which is distinct from an empty args object — the catalog schemas reject the latter for functions that require arguments. Argument arity and types are checked against the catalog, not here.