Re: [PATCH] Define alignof_slot using _Alignof when using C11 or newer

[Paul Eggert]

On 2023-01-14 16:17, Bruno Haible wrote:
> What's (quasi) normative is
>
>    * n3047.pdf = C23, which says in 7.21.(3)
>      "If the specified type defines a new type or if the specified member is
>       a bit-field, the behavior is undefined."

I'm afraid this wording in n3047 is unclear. Ordinarily one would look 
at n3047's section "Type definitions" to interpret the phrase "defines a 
new type", but as that section merely talks about typedefs it doesn't 
seem to be relevant here. I can't find any part of the standard that 
talks about "defining a new type".

All the standardizers wanted here is for the TYPE to not contain a 
top-level comma in 'offsetof (TYPE, MEMBER)' because that would screw up 
macro argument identification.

I hope Clang 16 is not reporting an error for 'offsetof (struct {char a; 
int b;}, b)'; if so, it's gone off the deep end. A nameless struct does 
not "define a new type" for some reasonable definitions of the term 
"define a new type", and since the standard doesn't define that term the 
Clang developers should be cautious about diagnostics in this messy area 
when a lot of code out there uses offsetof in precisely this way.