[tiffanyh]

Using the midpoint function is the most readable.

https://en.cppreference.com/w/cpp/numeric/midpoint

Though it's odd this allows overflow.

[abainbridge]

std::midpoint also seems to yield less efficient code with g++12 targeting x86-64: https://godbolt.org/z/j695ce98Y

[Lockal]

std::midpoint does different thing in terms of rounding, therefore comparison is not fair.