gistfile1.txt

class Foo {
  Foo operator << (_) => new Foo();
  Foo operator <= (_) => new Foo();
  
  // For the last case below
  Foo operator == (_) => new Foo();
}

main(List<String> arguments) {
  var foo = new Foo();

  // This works
  foo << 1 << 2 << 3 << 4;

  // This fails to parse
  // foo <= 1 <= 2 <= 3 <= 4;
  
  // I can add brackets to make the parser happy
  (((foo <= 1) <= 2) <= 3) <= 4;
  
  // But why?
  
  // Also, why is this fine?
  foo <= 2 == 3 < 4;
}

Line 16 fails to parse because, unlike additiveExpression, which is defined to allow multiple operators:

additiveExpression:
    multiplicativeExpression (additiveOperator multiplicativeExpression)*

relational expressions are not:

relationalExpression:
    bitwiseOrExpression (typeTest | typeCast | relationalOperator bitwiseOrExpression)?