bool fun(){
  int a = 0, b, c  = 0;
  b =  ++a;
  return c = a++;
}
bool r = !fun();

The current wording does not seem to specify the value computation of the function call expression fun() and the evaluation of expressions within the function body(e.g. the side effect of expression a). Every evaluation of the expressions before return statement ought to be guaranteed to be sequenced before the value computation of the function call. There is no explicit wording for this point. The relevant wording about the sequence for function call are defined in [expr.call] and [basic.exec]
[expr.call] p8

The postfix-expression is sequenced before each expression in the expression-list and any default argument. The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.

[intro.execution] p11

When invoking a function (whether or not the function is inline), every argument expression and the postfix expression designating the called function are sequenced before every expression or statement in the body of the called function. For each function invocation or evaluation of an await-expression F, each evaluation that does not occur within F but is evaluated on the same thread and as part of the same signal handler (if any) is either sequenced before all evaluations that occur within F or sequenced after all evaluations that occur within F;39 if F invokes or resumes a coroutine ([expr.await]), only evaluations subsequent to the previous suspension (if any) and prior to the next suspension (if any) are considered to occur within F.

Neither of them can clarify the above issue.