Ambiguity with regular expressions and comments

[sungshik]

Describe the bug

This is not ambiguous:

if (
//  /0  := [1,2,3]
    /0 !:= [1,2,3]
) {
    println("foo");
} else {
    println("bar");
}

But, this is ambiguous:

if (
    /0  := [1,2,3]
//  /0 !:= [1,2,3]
) {
    println("foo");
} else {
    println("bar");
}

The parser seems to be in doubt about whether:

• /0 := [1,2,3]\n/ is a regular expression, or
• // /0 !:= [1,2,3] is a comment.

Stack traces

rascal>if (
>>>>>>>    /0  := [1,2,3]
>>>>>>>//  /0 !:= [1,2,3]
>>>>>>>) {
>>>>>>>    println("foo");
>>>>>>>} else {
>>>>>>>    println("bar");
>>>>>>>}
Unexpected failure during parsing of current command:
Ambiguous code (internal error), if (
    /0  := [1,2,3]
//  /0 !:= [1,2,3]
) {
    println("foo");
} else {
    println("bar");
}
org.rascalmpl.interpreter.asserts.Ambiguous: Ambiguous code (internal error), if (
    /0  := [1,2,3]
//  /0 !:= [1,2,3]
) {
    println("foo");
} else {
    println("bar");
}
        at org.rascalmpl.parser.uptr.UPTRNodeFactory.buildAmbiguityNode(UPTRNodeFactory.java:76)
        at org.rascalmpl.parser.uptr.UPTRNodeFactory.createAmbiguityNode(UPTRNodeFactory.java:85)
        at org.rascalmpl.parser.uptr.UPTRNodeFactory.createAmbiguityNode(UPTRNodeFactory.java:23)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:183)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.buildAlternative(SortContainerNodeFlattener.java:90)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:65)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherAlternatives(SortContainerNodeFlattener.java:53)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:166)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.buildAlternative(SortContainerNodeFlattener.java:90)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:65)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:71)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherProduction(SortContainerNodeFlattener.java:71)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.gatherAlternatives(SortContainerNodeFlattener.java:53)
        at org.rascalmpl.parser.gtd.result.out.SortContainerNodeFlattener.convertToUPTR(SortContainerNodeFlattener.java:166)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:63)
        at org.rascalmpl.parser.gtd.result.out.DefaultNodeFlattener.convert(DefaultNodeFlattener.java:79)
        at org.rascalmpl.parser.gtd.SGTDBF.buildResult(SGTDBF.java:1306)
        at org.rascalmpl.parser.gtd.SGTDBF.parse(SGTDBF.java:1245)
        at org.rascalmpl.parser.gtd.SGTDBF.parse(SGTDBF.java:1249)
        at org.rascalmpl.parser.gtd.SGTDBF.parse(SGTDBF.java:1261)
        at org.rascalmpl.interpreter.Evaluator.parseCommand(Evaluator.java:1052)
        at org.rascalmpl.interpreter.Evaluator.parseCommand(Evaluator.java:1042)
        at org.rascalmpl.repl.RascalInterpreterREPL.isStatementComplete(RascalInterpreterREPL.java:176)
        at org.rascalmpl.repl.BaseRascalREPL.handleInput(BaseRascalREPL.java:119)
        at org.rascalmpl.vscode.lsp.terminal.LSPTerminalREPL$1.handleInput(LSPTerminalREPL.java:246)
        at org.rascalmpl.repl.BaseREPL.handleInput(BaseREPL.java:179)
        at org.rascalmpl.repl.BaseREPL.run(BaseREPL.java:346)
        at org.rascalmpl.vscode.lsp.terminal.LSPTerminalREPL.main(LSPTerminalREPL.java:333)

[jurgenvinju]

Good catch!