Source generators does not account for visibility of lexers.

[kant2002]

internal enum ТокенГакст {}
[Lexer(typeof(ТокенГакст))]
public partial class ЛексерГакст { }

produce

partial class ЛексерГакст { 
   public Token<GAXT.NET.ТокенГакст> Next() ..
}

and that's compile time error, since ТокенГакст is internal, and thus cannot be public method.