Idiom exclusion is really so important

Half of the time you want to both add and bail. Another half is to only check because operation in question will be done much later
and passing the result is cumbersome or not necessary.

The _right_ way is to not invent new idioms (which every project will do differently) but to use __builtin_add_overflow().

T len;
if (__builtin_add_overflow(a, b, &len)) { return -E; };
[use len]

or (because clang doesn't do BAO_p)

if (__builtin_add_overflow(a, b, &(int[1]){})) { return -E; };

Those who fear that BAO does addition differently and will make people break glass and cut hands may want to lobby for a warning if BAO is done on different types (especially with both different signedness _and_ length).

BAO is cool for checking if result fits into some other type:

if (__builtin_add_overflow(a, 0, &(T[1]){})

Just use BAO.