UnknownAlias:`Typer: unknown type alias in "${S}" — register it with extend() or fix the spelling`
The type a schema slot is reported as when it names an alias that is not
registered. Surfacing it as a string literal makes TypeScript print the
offending alias in the assignability error.
The type a schema slot is reported as when it names an alias that is not registered. Surfacing it as a string literal makes TypeScript print the offending alias in the assignability error.