Array access when using nested arrays

[Mat]

If arrays are nested then it is not possible to access them using brackets directly.

Local $a1[3] = [0, 0, 0]
Local $a2[3] = [1, 2, 3]

$a1[2] = $a2

;MsgBox(0, "Test", ($a1[2])[1]) ; Works
MsgBox(0, "Test", $a1[2][1]) ; Doesn't

Furthermore, using parenthesis like above to try and assign to that does nothing at all (no error, no effect):

#include<Array.au3>


Local $a1[3] = [0, 0, 0]
Local $a2[3] = [1, 2, 3]

$a1[2] = $a2

($a1[2])[1] = 42

_ArrayDisplay($a1[2])

[trancexx]

That's how it's supposed to be, because that's how array access is docummented to be.
If former would work there would be no way to make distinction between nested 1D array and regular 2D array, which would be wrong.
Latter is ok too. The line of your code before the last one is stateless expression which evaluates to False, meaning it's not an assignment.