`index[array]` は C および C の有効な配列アクセス構文ですか?

int arr[] = {1, 2, 3};
2[arr] = 5; // Does this compile?
assert(arr[2] == 5); // Does this assertion fail?

One of the expressions shall have type "pointer to object type", the other expression shall have integer type, and the result has type "type".

A postfix expression followed by an expression in square brackets [] is a subscripted designation of an element of an array object. The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).