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?

Jason