Glad assume(expr) is available, if it maps onto builtin-expect but confused by the UB reference. Why would taking the less likely path be undefined behaviour?

Glad assume(expr) is available, if it maps onto builtin-expect but confused by the UB reference. Why would taking the less likely path be undefined behaviour? Edit. It's because assume does not map to expect, it maps to builtin_assume. So that's just another way to write undefined c++.