On 15 Oct 2025, at 15:20, Ran Regev <regev.ran@gmail.com> wrote:

On Wed, Oct 15, 2025, 14:10 Timur Doumler via SG15 <sg15@lists.isocpp.org> wrote:

Specifically about the inconsistency that Tom highlighted:

Please correct me if I'm wrong, but if I remember correctly, we discussed this point specifically when reviewing the P3471 wording in CWG+LWG. From what I remember, the outcome of that discussion was that we can (and did) write in the wording that the standard library is using contract assertions as defined in the core language (`pre`, `post`, and `contract_assert`) but that does not actually imply that those keywords are physically present in the standard library implementation, only that the standard library behaves as if they were there.

In other words, if a standard library implementation internally uses macros — or some other implementation-defined mechanism — instead of the contextual keywords  `pre`, `post`, and `contract_assert`, then that implementation is still conforming with that wording as long as the observable behaviour is the same as if those keywords were used —

P2900 suggests that the contract semantics are selected by the one that is responsible for building the application, not the library writer. 

If the library uses a different tool that is not the contextual keywords, and thus allows different semantics for the library than for the application - is this within the "observable behaviour is the same as if those keywords were used"?

Assuming that by "library", you mean "standard library":

The mechanism for choosing the evaluation semantics of any particular evaluation of any assertion anywhere in the program is implementation-defined. It follows that allowing different contract-evaluation semantics for the standard library than for the rest of the program would be conforming. It follows further that offering one flag for choosing the contract-evaluation semantics of the standard library and a different flag for the contract-evaluation semantics of the rest of the program would be conforming too.

Does that answer your question?

Timur