3 Terms and definitions [intro.defs]

3.50[defns.undefined.runtime]runtime-undefined behavior

behavior that is undefined except when it occurs during constant evaluation
[Note 1: 
During constant evaluation,
  • it is implementation-defined whether runtime-undefined behavior results in the expression being deemed non-constant (as specified in [expr.const]) and
  • runtime-undefined behavior has no other effect.
— end note]