Function reference_is_verifiable 

fn reference_is_verifiable(reference: &UnresolvedItemName) -> bool

Whether the recorded references can settle reference at all.

MySQL’s system schemas are the blind spot. Both CREATE SOURCE and ALTER SOURCE ... REFRESH REFERENCES retrieve MySQL tables with system schemas excluded, so mz_source_references never lists a table in mysql, sys, performance_schema, or information_schema. Creating a table from such a reference does resolve it, so only the server can judge one, and reporting it missing here would block a deploy that works.

The namespace alone decides this, without consulting the source’s connection type. A Postgres or SQL Server schema that happens to be named mysql or sys is skipped too, which costs nothing beyond leaving those references to the server.