I think we assume poison, not undef, for certain transforms we currently do. In any case, we should clarify the language here.
The C++ standard defines this as UB: http://eel.is/c++draft/conv.fpint#1
"A prvalue of a floating-point type can be converted to a prvalue of an integer type. The conversion truncates; that is, the fractional part is discarded.
The behavior is undefined if the truncated value cannot be represented in the destination type."