It's mathematically correct, but integers use truncating (floor) division. So you can lose a fractional 0.5 from both a/2 and b/2, which would have added up to 1, and now your result is off by one from (a+b)/2.

In general, even with floating point numbers you can get different results by rounding implicitly or intentionally the intermediate values versus the end result.

If a=1 and b=1, the midpoint should be 1.

But in computing, assuming we are only dealing with integers, 1/2 == 0. So 1/2 + 1/2 returns 0.

The dev blog from Microsoft in a different comment covers this:

"This will be too low if the original addition contained a carry from bit 0 to bit 1, which happens if bit 0 is set in both of the terms, so we detect that case and make the necessary adjustment."

return (a / 2) + (b / 2) + (a & b & 1);

If a & b are both odd, you end up rounding twice vs. 0 times if you take the sum first.

Because, assuming / means integer division, this would return 6 as the midpoint between 7 and 7.