What happens to unused function return values?

Since you're talking about Windows, we'll assume an x86 processor.

In this case, the return value will typically be in register EAX. Since you're not using it, that value will simply be ignored, and overwritten the next time some code that happens to write something into EAX executes.

In a fair number of cases, if a function has no other side effects (just takes inputs and returns some result) the compiler will be able to figure out when you're not using the result, and simply not call the function.

In your case, the function has some side effects, so it has to carry out those side effects, but may well elide code to compute the sum. Even if it wasn't elided, it can probably figure out that what's being added are really two constants, so it won't do an actual computation of the result at run-time in any case, just do something like mov EAX, 2000 to produce the return value.

It is discarded; the expression TestInReturn(a,b) is a discarded-value expression. Discarding an int has no effect, but discarding a volatile int (or any other volatile-qualified type) can have the effect of reading from memory.

The return value simply gets discarded. Depending on the exact scenario the optimizer might decide to optimize away the whole function call if there are no observable side effects (which is not the case in your example).

So, upon return from TestIntReturn, the function will push the return value on the stack, the caller will then adjust the stack frame accordingly, but won't copy the returned value from the stack into any variable.