Robin isn't ignoring those important words; he specifically accounted
for them by invoking implementation-specific definitions of behavior.
Perhaps, but I interpreted the balance of his remark as implying that
it wasn't undefined if an implementation defined it. This is wrong.
So even if an implementation defines the behavior of signed integer
overflow as the usual 2's-complement wraparound, the behavior is still
undefined. Strictly speaking, it's not even implementation-defined
behavior; "implementation-defined behavior" is a subset of
"unspecified behavior", and integer overflow doesn't meet that
The areas of doubt and uncertainty around implementation defined are
even more rigdly defined than those around UB.
Was Douglas a committe member by the way?