int said:
That is undefined because neither assignment
is guaranteed to take place before the other.
The expression (x = 20), can be evaluated as 20,
and the sum of 20 and x can be assigned to x,
prior to the side effect of the assignment taking place
in the (x = 20) expression.
The c code statement is modifying an object twice
without an intervening sequence point.
ISO/IEC 9899:1999
6.5 Expressions
2 Between the previous and next sequence point
an object shall have its stored value modified
at most once by the evaluation of an expression.
Furthermore, the prior value shall be read only to
determine the value to be stored.
6.8 Statements and blocks
4 A full expression is an expression
that is not part of another expression or of a declarator.
Annex C
(informative)
Sequence points
1 The following are the sequence points described in 5.1.2.3:
— The call to a function,
after the arguments have been evaluated (6.5.2.2).
— The end of the first operand of the following operators:
logical AND && (6.5.13);
logical OR || (6.5.14);
conditional ? (6.5.15);
comma , (6.5.17).
— The end of a full declarator: declarators (6.7.5);
— The end of a full expression: an initializer (6.7.8);
the expression in an expression statement (6.8.3);
the controlling expression of a selection statement
(if or switch)(6.8.4);
the controlling expression of a while or do statement (6.8.5);
each of the expressions of a for statement (6.8.5.3);
the expression in a return statement (6.8.6.4).
— Immediately before a library function returns (7.1.4).
— After the actions associated with each formatted
input/output function conversion specifier (7.19.6, 7.24.2).
— Immediately before and immediately after each call
to a comparison function, and also between any call
to a comparison function and any movement of the objects
passed as arguments to that call (7.20.5).