Could a C++ implementation, in theory, parallelise the evaluation of two function arguments?

Question

Given the following function call:

f(g(), h())

since the order of evaluation of function arguments is unspecified (still the case in C++11

Answer 1

The requirement comes from [intro.execution]/15:

... When calling a function ... Every evaluation in the calling function (including other function calls) that is not otherwise specifically sequenced before or after the execution of the body of the called function is indeterminately sequenced with respect to the execution of the called function [Footnote: In other words, function executions do not interleave with each other.].

So any execution of the body of g() must be indeterminately sequenced with (that is, not overlapping with) the evaluation of h() (because h() is an expression in the calling function).

The critical point here is that g() and h() are both function calls.

(Of course, the as-if rule means that the possibility cannot be entirely ruled out, but it should never happen in a way that could affect the observable behaviour of a program. At most, such an implementation would just change the performance characteristics of the code.)