On 21/01/2023 18:19, Edward Catmur wrote:
> On Sat, 21 Jan 2023, 13:09 , <language.lawyer_at_[hidden]> wrote:
>> On 21/01/2023 18:01, Edward Catmur wrote:
>>> On Sat, 21 Jan 2023, 12:57 , <language.lawyer_at_[hidden]> wrote:
>>>> On 21/01/2023 17:49, Edward Catmur wrote:
>>>>> On Sat, 21 Jan 2023, 12:40 , <language.lawyer_at_[hidden]> wrote:
>>>>>> On 21/01/2023 17:37, Edward Catmur wrote:
>>>>>>> On Sat, 21 Jan 2023, 12:34 , <language.lawyer_at_[hidden]> wrote:
>>>>>>>> On 21/01/2023 17:31, Edward Catmur wrote:
>>>>>>>>> On Sat, 21 Jan 2023, 12:25 language.lawyer--- via Std-Proposals, <
>>>>>>>>> std-proposals_at_[hidden]> wrote:
>>>>>>>>>> On 20/01/2023 12:42, Jens Maurer via Std-Proposals wrote:
>>>>>>>>>>> Anyway, the "i++" is undefined behavior per [expr.add] p6:
>>>>>>>>>>>
>>>>>>>>>>>> For addition or subtraction, if the expressions P or Q have type
>>>>>>>>>> “pointer to cv T”,
>>>>>>>>>>>> where T and the array element type are not similar (7.3.6), the
>>>>>>>>>> behavior is undefined.
>>>>>>>>>>
>>>>>>>>>> In f(b), first we get array-to-pointer conversion to get a pointer
>>>> to
>>>>>>>> the
>>>>>>>>>> first array element, and then conversion to pointer to its I base
>>>>>> class
>>>>>>>>>> subobject, so the pointer type and the object it points to are
>>>>>> aligned.
>>>>>>>>>> Which means there is no UB per [expr.add]/6.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> "similar" isn't specified in terms of alignment.
>>>>>>>>
>>>>>>>> "aligned" == "coherent/matching/in agreement"
>>>>>>>>
>>>>>>>
>>>>>>> I have no idea what you mean by that. Are you saying that the array
>>>>>> element
>>>>>>> type and the base class type are similar?
>>>>>>
>>>>>> I mean that doing pointer arithmetic on a «pointer to I» which points
>> at
>>>>>> object of type I is OK from [expr.add]/6 POV.
>>>>>>
>>>>>
>>>>> The base class subobject and pointer type are similar, but the array
>>>>> element type is not.
>>>>
>>>> How can T in «pointer to T» expression type be similar to the type of
>> the
>>>> base class subobject, and not similar to the «array element type» at the
>>>> same time, when array element here is the base class subobject?
>>>>
>>>
>>> The array element type is B.
>>
>> How `b` array's element type is relevant here? We do not use a pointer to
>> `b`'s element.
>>
>
> Yes we do. The pointer being incremented points to an array of element type B.

The pointer points neither to an array nor to an object of type B.

> Which array of element type I do you identify in the program?
https://timsong-cpp.github.io/cppwp/n4861/basic.compound#3.sentence-12, no? [expr.add] even has a reminder about it (https://timsong-cpp.github.io/cppwp/n4861/expr.add#footnote-76)