During the review of [P0556R1], LEWG requested
return an unspecified
value for out-of-bounds arguments. [P0556R3] was adopted in Rapperswil with
this specification. [N4778] (24.6.4 "Integral powers of 2"):
Returns: The minimal value
ispow2 ( y )
y >= x
is not representable as a value of type
, the result is an unspecified value.
At the [SAN] meeting, LEWG took a poll reconsidering this decision at the request of SG6:
Change ceil2 to hard UB, ill-formed in constexpr
SF F N A SA 11 5 1 0 0
As discussed in [P1233R0] (out-of-bounds
silently accepting out-of-bounds values can hide bugs. Making this explicitly
undefined behavior allows it to be detected and flagged by analysis tools.
Wording relative to [N4778]:
template < class T > constexpr T ceil2 ( T x ) noexcept ;
Returns: The minimal value y such that ispow2(y) is true and y >= x
; if y is not representable as a value of type T, the result is an unspecified value.
Remarks: This function shall not participate in overload resolution unless T is an unsigned integer type ([basic.fundamental]).
For values of
that fail to satisify the Expects precondition, this
results in undefined behavior (18.104.22.168).