Misplaced parenthesis in object destructuring argument with trailing argument comma

[tlrobinson]

I haven't been able to pin down exactly why this is happening, but here's my smallest reproduction so far. I'm using flow the parser.

class Foo {
  method = item => {
    return (
      <div className=""></div>
    );
  };
}

function x({ className, }) {
}

gets printed as

class Foo {
  method = item => {
    return (
      <div className=""></div>
    );
  };
}

function x({ (className, })) {
}

Note the extra (and mis-nested) parenthesis in the last function.

Interestingly, changing just about anything about the code above causes the problem to go away.

[benjamn]

This is just a guess, but in the past, bugs like this have sometimes been due to the parser providing slightly incorrect .loc.{start,end}.{line,column} information. Like, perhaps the parser gets confused by the trailing comma and decides that the className identifier location extends all the way past the end of the parameter list… or something like that.

[benjamn]

Can you still reproduce this? I've been trying to write a regression test, and it seems to work now. If my previous theory is correct, the Flow parser might have fixed their .loc tracking in the meantime.