“defined behavior – (1) behavior that is unambiguously codified in terms of C++’s abstract machine or (2) the full set of behaviors defined for a given component or library. Note that invoking a component or library out of contract is library undefined behavior (a.k.a. soft UB), which might lead to language undefined behavior (a.k.a. hard UB). noexcept Specifier (1112)” (EMCppSfe 2021)