Right. And that's a mistake that can be made today without any ODR
violation.

Which is why I don't see this as a novelty of contracts, but I do concede
that contracts may increase the exposed surface area.

O.

On Mon, 20 Oct 2025 at 16:32, Tom Honermann <tom_at_[hidden]> wrote:

> If someone builds part of their program with one compile-time selected
> semantic and another part with a different compile-time selected semantic
> and then expects a consistent semantic to govern their program, I would
> argue that they made a mistake.
>
> Tom.
> On 10/20/25 11:28 AM, John Spicer via SG15 wrote:
>
> Yes, there are lots of ways people can make mistakes and get strange
> things.
>
> The difference with contracts is you get strange things without making
> mistakes.
>
> John.
>
> On Oct 20, 2025, at 11:26 AM, Oliver Rosten via SG21
> <sg21_at_[hidden]> <sg21_at_[hidden]> wrote:
>
> But I don't think it's true that it's all plain-sailing today.
>
> You can compile one TU with fast-math and one without. The header code
> they consume is token-identical.
>
> There's no ODR violation but you can get differences depending on which
> version the linker chooses. And these differences need not be small. If the
> result of a floating-point calculation is used to determine if a grid cell
> is open or closed, tiny numerical differences can blow up into boolean
> yes/no differences.
>
> Isn't this something the community already lives with? Contracts may
> increase the surface area for such things but I don't think it's novel.
>
> O.
>
> On Mon, 20 Oct 2025 at 16:19, John Spicer <jhspicer_at_[hidden]> wrote:
>
>> By “do everything right” I meant “you don’t have an ODR violation”.
>>
>> Not that your program is perfect.
>>
>> I agree that in perfect programs you don’t need contracts.
>>
>> But for every actual program, people want contracts do what they can
>> reasonably expect that they will do.
>>
>> John.
>>
>> On Oct 20, 2025, at 11:16 AM, Oliver Rosten via SG21 <
>> sg21_at_[hidden]> wrote:
>>
>> Hi John,
>>
>> Contracts add the new issue that you can do everything right and still
>>> get surprising behavior
>>
>>
>> If you've done everything right then contracts are completely redundant
>> in your program.
>>
>> O.
>>
>>
>> On Mon, 20 Oct 2025 at 16:14, John Spicer <jhspicer_at_[hidden]> wrote:
>>
>>> In general, if you do things correctly, you don’t need to understand ODR
>>> violations.
>>>
>>> If you do things incorrectly, it might be hard to figure out. But having
>>> header files mean different things in difference places is a long-standing
>>> issue that goes back to C and has always been hard to figure out.
>>>
>>> Contracts add the new issue that you can do everything right and still
>>> get surprising behavior and it is even harder to figure out because your
>>> tools won’t help you.
>>>
>>> John.
>>>
>>> On Oct 20, 2025, at 9:56 AM, Oliver Rosten <oliver.rosten_at_[hidden]>
>>> wrote:
>>>
>>> PS Most people I teach have no prior idea what an ODR violation is.
>>>
>>> On Mon, 20 Oct 2025 at 14:55, Oliver Rosten <
>>> oliver.rosten_at_[hidden]> wrote:
>>>
>>>> That's a proof of existence but not of wide-spread usage.
>>>>
>>>> I am honestly ignorant here. As far as I know the C++ ecosystem as a
>>>> whole is not making rigorous use of things like this. But I may be wrong...
>>>>
>>>> On Mon, 20 Oct 2025 at 14:53, Ville Voutilainen <
>>>> ville.voutilainen_at_[hidden]> wrote:
>>>>
>>>>> On Mon, 20 Oct 2025 at 16:46, Oliver Rosten
>>>>> <oliver.rosten_at_[hidden]> wrote:
>>>>> >
>>>>> > Hi John,
>>>>> >
>>>>> > I'm not convinced by this:
>>>>> >
>>>>> >> No, it is not a pre-existing problem.
>>>>> >> Other than contracts, if you end up with different function
>>>>> definitions it is an ODR violation and your program is IFNDR and can be
>>>>> rejected by your tools.
>>>>> >
>>>>> >
>>>>> > There's a difference between "can be in principle" and "is in
>>>>> general practice". Is it not the case that, in most instances, for all
>>>>> practical purposes there is no difference between an ODR violation that's
>>>>> IFNDR and the contracts mixed-mode: you get what the linker gives you?
>>>>>
>>>>> See, for example,
>>>>> https://devblogs.microsoft.com/oldnewthing/20160803-00/?p=94015
>>>>> See also https://maskray.me/blog/2022-11-13-odr-violation-detection
>>>>>
>>>>
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