1. Background
During the review of [P0556R1], LEWG requested
return an unspecified
value for outofbounds arguments. [P0556R3] was adopted in Rapperswil with
this specification. [N4791] ([bit.pow.two] 25.5.4 "Integral powers of 2"):
Returns: The minimal value
such that
y is
ispow2 ( y ) true
and; if
y >= x is not representable as a value of type
y , the result is an unspecified value.
T
At the [SAN] meeting, LEWG took a poll reconsidering this decision at the request of SG6:
Change ceil2 to hard UB, illformed in constexpr
SF F N A SA 11 5 1 0 0
As discussed in [P1233R0] (outofbounds
/
),
silently accepting outofbounds values can hide bugs. Making this explicitly
undefined behavior allows it to be detected and flagged by analysis tools.
2. Proposal
Wording relative to [N4791]:
template < class T > constexpr T ceil2 ( T x ) noexcept ;
 Let
be the smallest power of 2 greater than or equal to
N .
x  Expects:
is representable as a value of
N .
T Returns:
N The minimal valuesuch that
y is
ispow2 ( y ) true
and; if
y >= x is not representable as a value of type
y , the result is an unspecified value.
T  Constraints:
Remarks: This function shall not participate in overload resolution unlessis an unsigned integer type ([basic.fundamental]).
T  Remarks: Call expressions that violate the precondition in the Expects element are not core constant expressions ([expr.const]).
3. Discussion
For values of
that fail to satisify the Expects precondition, this
results in undefined behavior ([res.on.required] 15.5.4.11).
This function is intentionally
with a narrow contract:

is a lowlevel function.ceil2 
Reflector recollection of the San Diego meeting indicates this was the desire of LEWG, although minutes for this discussion are limited.
4. History
4.1. R0 → R1
Applied feedback from [PostSanDiegoReview].

Added wording suggested by Casey Carter.

Added clarification that having a narrow contract and
is intentional.noexcept