Fix for Issue 3319 #3322
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Fix for Issue 3319 #3322
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This is a fix for #3319 which is related to the problem reported here.
The problem is in the Go runtime, specifically with ATNConfigSet's. In August 2021, there was a commit for a new hash set implementation for sets, which exposed a bug in ATNConfigSet's that has existed for over 5 years. It was only now revealed due to the confluence of that commit, the huge test suite of Antlr4 grammars in github.com/antlr/grammars-v4 now ported to 6 or so targets, and a bug in the GitHub Actions build for grammars-v4! Our lucky day!
The August 2021 commit implements the hash set as a two-level hash table with probing. The problem occurs if one tries to add an ATNConfig to the table with a nil lexerActionExecutor field when the ATNConfig's collide in the hash table. This only happens when two states have equal state numbers module 16. The old code would get a segv at this line of code. The solution is to first cast "other" to "othert" and then test that it is nil prior to use.
I added code to test the bug in file atnconfigset_test.go with this change. (Please read the comments in the file to see how to test.)