Historically, "undefined behavior" was sometimes used for patterns that were supported on some platforms and not others.  Many of those have since become "conditionally supported", making the construct either well-formed with defined behavior or ill-formed.  But what about patterns that are well-defined on some platforms and undefined on others?

I'm thinking specifically about floating point division by zero, which sometimes hits a hardware exception and sometimes produces floating-point infinity (when is_iec559 is true).

As with other cases of undefined behavior, this is made testable by constexpr.  Can an implementation say that a construct that is undefined in the C++ standard is defined in that implementation, and therefore accept it in constexpr?