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.

> The base class subobject type is I.

And the pointer pointing to it has type `I*`. I == I. [expr.add]/6 is happy.