7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.1 Postfix expressions [expr.post] Subscripting [expr.sub]

A subscript expression is a postfix expression followed by square brackets containing a possibly empty, comma-separated list of initializer-clauses that constitute the arguments to the subscript operator.
The postfix-expression and the initialization of the object parameter of any applicable subscript operator function is sequenced before each expression in the expression-list and also before any default argument.
The initialization of a non-object parameter of a subscript operator function S ([over.sub]), including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other non-object parameter of S.
With the built-in subscript operator, an expression-list shall be present, consisting of a single assignment-expression.
One of the expressions shall be a glvalue of type “array of T” or a prvalue of type “pointer to T” and the other shall be a prvalue of unscoped enumeration or integral type.
The result is of type “T.
The type “T” shall be a completely-defined object type.49
The expression E1[E2] is identical (by definition) to *((E1)+(E2)), except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise.
[Note 1: 
Despite its asymmetric appearance, subscripting is a commutative operation except for sequencing.
See [expr.unary] and [expr.add] for details of * and + and [dcl.array] for details of array types.
— end note]
This is true even if the subscript operator is used in the following common idiom: &x[0].