@maxonfjvipon Could you have a look, please?

@volodya-lombrozo it's a very interesting idea, but I'm not sure if it's "right" to put wrong scenarios to grammar to detect errors. It seems that grammar is supposed to contain only "possible" scenarios. Did you see anywhere else such approach when obviously incorrect scenarios are put into grammar to detect syntax errors?

@maxonfjvipon I share your concern. Actually, using this approach was a last resort. I didn’t want to use it initially.
As for examples, yes, there are plenty of them:

1. The Definitive ANTLR 4 Reference 2nd Edition: Section 9.4 Error Alternatives
2. The real example is the Groovy lang. They use this technique a lot (at least in Lexer). For example for UNEXPECTED_CHAR
3. Also some sites mention this approach as well

@volodya-lombrozo maybe we should use a two-phases compilation pipeline for the .g4 file. First, we take the original .g4 file and inject such additional constructs into it, producing new (very large) .g4 file. Then, we let ANTLR compile this large file. WDYT?