On 4/3/24 12:42, Jakub Jelinek wrote:
On Wed, Apr 03, 2024 at 12:07:48PM -0400, Jason Merrill wrote:
Using std::is_constant_evaluated directly in a loop condition is, as the
paper says, unlikely and "horrendous code", so I'm not concerned about
surprising effects, though I guess we should check for it with
maybe_warn_for_constant_evaluated.
Ok, though guess the question is what to say about it.
Because unlike the existing cases in maybe_warn_for_constant_evaluated
where it always evaluates to true or always to false depending on where,
in the trivial empty iteration statements it evaluates to always true or
always false depending or sometimes true, sometimes false, depending on
if the condition is a constant expression that evaluates to true (then it is
always true), or if in immediate function (also always true), or if not
in constexpr function (then always false), or in constexpr function (then
it might be true or false).
Not sure how exactly to word that.
Maybe just say that it is horrendous code to use std::is_constant_evaluated
() in trivial empty iteration statement conditions ;)
Maybe if the condition constant-evaluates to true, warn something like
"%<std::is_constant_evaluated%> always
constant-evaluates to true in the condition of a trivially empty
iteration statement"?
What about loops with non-empty bodies or other reasons why they aren't
trivial empty iteration statements? Shall we do
maybe_warn_for_constant_evaluated for those (with the current wording,
false in non-constexpr fn, true in consteval)?
Sounds good.
Jason
