Undefined behavior may be expected when
this document omits any explicit
definition of behavior or when a program uses an erroneous construct or erroneous data.
Permissible undefined behavior ranges
from ignoring the situation completely with unpredictable results, to
behaving during translation or program execution in a documented manner
characteristic of the environment (with or without the issuance of a
diagnostic message), to terminating a translation or execution (with the
issuance of a diagnostic message).
Many erroneous program constructs do
not engender undefined behavior; they are required to be diagnosed.
Evaluation of a constant expression never exhibits behavior explicitly
specified as undefined ([expr.const]
— end note