Re: [V1][PATCH 1/3] Provide element_count attribute to flexible array member field (PR108896)

[Qing Zhao <qing.zhao@oracle.com>]

> On Jun 16, 2023, at 3:21 AM, Martin Uecker <uecker@tugraz.at> wrote:
> 
> Am Donnerstag, dem 15.06.2023 um 16:55 +0000 schrieb Joseph Myers:
>> On Thu, 15 Jun 2023, Qing Zhao via Gcc-patches wrote:
>> 
> ...
>>> 1. Update the routine “c_parser_postfix_expression” (is this the right 
>>> place? ) to accept the new designator syntax.
>> 
>> Any design that might work with an expression is the sort of thing that 
>> would likely involve many iterations on the specification (i.e. proposed 
>> wording changes to the C standard) for the interpretation of the new kinds 
>> of expressions, including how to resolve syntactic ambiguities and how 
>> name lookup works, before it could be considered ready to implement, and 
>> then a lot more work on the specification based on implementation 
>> experience.
>> 
>> Note that no expressions can start with the '.' token at present.  As soon 
>> as you invent a new kind of expression that can start with that token, you 
>> have syntactic ambiguity.
>> 
>> struct s1 { int c; char a[(struct s2 { int c; char b[.c]; }) {.c=.c}.c]; };
>> 
>> Is ".c=.c" a use of the existing syntax for designated initializers, with 
>> the first ".c" being a designator and the second being a use of the new 
>> kind of expression, or is it an assignment expression, where both the LHS 
>> and the RHS of the assignment use the new kind of expression?  And do 
>> those .c, when the use the new kind of expression, refer to the inner or 
>> outer struct definition?
> 
> I would treat this is one integrated feature. Essentially .c is
> somthing like this->c for the current struct for designated
> initializer *and* size expressions because it is semantically 
> so close.  

Yes, I think this is reasonable. (Is “this” the immediate containing structure?)

>  In the initializer I would allow only 
> the current use for designated initialization for all names of
> member of the currently initialized struct,  so .c = .c would 
> be invalid.

Given “.c” basically is “this->c”, then .c = .c should be considered as
this->c = this->c, is such self-initialization allowed in C?

>   It should never refer to the outer struct if there
> is a member with the same name in the inner struct, i.e. the
> outside member is then hidden. 

Does the above mean:  if there is NO name conflicting, it could refer to a field of an outer struct?

Why this is allowed? Why just disallow all referring to the field of the outer structure since .c basically is this->c?
> 
> So this would be ok:
> 
> struct s1 { int d; char a[(struct s2 { int c; char b[.c]; }) {.c=.d}.c]; };
> 
> Here the use of .d would be ok because it is not from the struct
> currently initialized, but from an outside scope.

I think that the above .c=.d should report an error, since .d does not exist in the containing structure.

Do I miss anything here?

thanks.

Qing
> 
> Martin
> 
> 
> 
>