But the square brackets used in array access nest, so the parsing is never ambiguous.

The reason > needs braces is that without them, if you're parsing and so far you've seen these characters:

  let y: RectangularArray<i32, X, 2>

You don't know if that final > is ending the argument list, or is a greater-than operator, with the rest of an expression coming after it.

But if it was square brackets, then a closing square bracket on its own can only ever be the end of the list:

  let y: RectangularArray[i32, X, 2]

Because if it was closing an array access, there would have to have been an opening square bracket:

  let y: RectangularArray[i32, X, a[1]