On 20 Oct 2025, at 13:32, Ville Voutilainen <ville.voutilainen@gmail.com> wrote:

On Mon, 20 Oct 2025 at 14:29, Timur Doumler via SG15
<sg15@lists.isocpp.org> wrote:

Hi John,

I agree that the problem you highlighted exists. I don't think anybody dismisses the existence of the problem. However, as described in P3846, the fundamental cause of the problem is the way in which the C++ compilation model works, and not anything specific to contracts, and in fact *any* feature that lets you change the semantic of code at build time inevitably runs into the same problem (and many existing macros and semantic-modifying compiler flags do have that problem today and are stil being widely used).

Now, as described in P3846, there are only three ways to address the problem:

(1) Accept that the problem exists and standardise P2900 anyway because it provides value to users despite the existence of the problem,

(2) Change P2900 to require deterministic semantics across TUs, which means the feature will not work with existing toolchains, which in turn means nobody will adopt it,

(3) Not standardise P2900 or any other such feature at all.

P3846 is wrong, if it suggests that there's only three ways to address
the problem. That's missing a

4) provide an assertion that cannot change its meaning based on build flags.