P1355R1
Exposing a narrow contract for ceil2

1. Background

During the review of [P0556R1], LEWG requested ceil2 return an unspecified value for out-of-bounds arguments. [P0556R3] was adopted in Rapperswil with this specification. [N4791] ([bit.pow.two] 25.5.4 "Integral powers of 2"):

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.

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 shift_left / shift_right), silently accepting out-of-bounds 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 N be the smallest power of 2 greater than or equal to x.
• Expects: N is representable as a value of T.

• Returns: N 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.

• Constraints: Remarks: This function shall not participate in overload resolution unless T is an unsigned integer type ([basic.fundamental]).
• Remarks: Call expressions that violate the precondition in the Expects element are not core constant expressions ([expr.const]).

3. Discussion

For values of x that fail to satisify the Expects precondition, this results in undefined behavior ([res.on.required] 15.5.4.11).

This function is intentionally noexcept with a narrow contract:

• ceil2 is a low-level function.

• 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 noexcept is intentional.