• I am not submitting a question on how to use ANTLR; instead, go to antlr4-discussion google group or ask at stackoverflow
• I have done a search of the existing issues to make sure I'm not sending in a duplicate

Reproducing Error

Running via Xcode for iOS 13
Project Swift Version: 4.2
Compiled swift target from 4.9.2 release

Grammar

grammar math;

NUMBER : [0-9] ;
WS : [ \r\n\t] + -> skip ;

operation : l=NUMBER op='+' r=NUMBER  ;

Swift Code

for _ in 0...10 {
    DispatchQueue.global().async {
        let chars = ANTLRInputStream("1+1")
        let lexer = mathLexer(chars)
        
        let tokenStream = CommonTokenStream(lexer)
        let parser = try! mathParser(tokenStream)
        
        let node = try? parser.operation()
    }
}

What Happened

The program crashes in LexerATNSimulator.swift on line 721 with EXC_BAD_ACCESS. There was a double free at some point in the stack.

Any help is greatly appreciated! Hopefully this is just user error.