(line 37,col 15) '_' is a reserved keyword.
(line 37,col 18) '_' is a reserved keyword.
(line 42,col 65) '_' is a reserved keyword.
(line 42,col 68) '_' is a reserved keyword.
(line 298,col 47) '_' is a reserved keyword.
This is a bug that was fixed but maybe not fully as your use case shows. If your Java version allows the keyword then Copilot should parse it correctly.
Could you open a ticket in Issues · vaadin/copilot · GitHub so we can investigate?
If possible with an example and dev env information such as Java Version used and Vaadin/Copilot versions?