Comment by 112233

Closest to what you say that I can find is 5.1.2.3 §4 of N3096

    In the abstract machine, all expressions are evaluated as specified by the semantics. An actual
    implementation need not evaluate part of an expression if it can deduce that its value is not used
    and that no needed side effects are produced (including any caused by calling a function or through
    volatile access to an object)

Problem is, calling external library function has a needed side effect of calling that library function. I do not see language that allows simply not doing that, based on assumed but unknown function behaviour.