[GROOVY-10236] Parser fails to parse a single-statement lambda when omitting parentheses for method call

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.0-beta-1
• Fix Version/s: 4.0.0-beta-2
• Component/s: parser, parser-antlr4
• Assignee: Daniel Sun
• Reporter: Lyuben Atanasov
• Created: 15/Sep/21 13:49
• Updated: 03/Feb/22 22:32
• Resolved: 16/Sep/21 02:58

Description

The parser fails to parse the following code:

class Test {

    public void test() {
        [1, 2, 3, 4, 5].stream().forEach(num -> println "$num");
    }

}

The produced error is:

Exception in thread "main" org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
Script_30c2a85fbeaac50f95df1e8ce776b5dd.groovy: 4: Unexpected input: '(' @ line 4, column 41.
   , 2, 3, 4, 5].stream().forEach(num -> pr
                                 ^

This problem appears only after 4.0.0-beta-1.
The following expressions are parsed successfully:

[1, 2, 3, 4, 5].stream().forEach(num -> { println "$num" });
[1, 2, 3, 4, 5].stream().forEach(num -> println("$num"));

There is also something curious: if I modify the original code and wrap the parameter num in parentheses (.forEach((num) -> println "$num")), the parser does not complain, but println is recognized as a variable rather than a method.