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. Which array of element type I do you identify in the program?

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