Consider this example

   int a[2][3] = {{0,1,2},{3,4,5}};
   int i = 0;
   auto r = a[i++][i++];

[expr.sub] p1 says

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.

In this example, a is sequenced before the first i++, and the a[i++] is sequenced before the second i++. [intro.execution] p8 says

An expression X is said to be sequenced before an expression Y if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y.

Do these two rules guarantee the side effect of the first i++(which is the subexpression of a[i++]) is sequenced before the value computation of the second i++? The meaning of the "associated with" is not clear enough here.

Improvement

An expression X is said to be sequenced before an expression Y if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y (including these of the subexpressions thereof, if any).

With this improvement, the side effect of the subexpressions is ascribed to the evaluation of the post-expression.