On Tue, 7 Feb 2023, Qing Zhao via Gcc-patches wrote:

> 1.  Structure with flexible array member embedded into other structures 
> recursively, for example:
> 
> struct A {
>   int n;
>   char data[];
> };
> 
> struct B {
>   int m;
>   struct A a;
> };
> 
> struct C {
>   int q;
>   struct B b;
> };
> 
> In the above, “struct C” will not be caught by this routine.

Because struct B is diagnosed with -pedantic when it embed struct A, there 
is no need for -pedantic to diagnose struct C as well when it embeds 
struct B.

> 2. Only C99 standard flexible array member be included, [0] and [1] are 
> not included, for example:

Obviously we can't diagnose use of structures with [1] trailing members, 
because it's perfectly valid to embed those structures at any position 
inside other structures.  And the same is the case for the [0] extension 
when it's used to mean "empty array" rather than "flexible array".

Note that my comments above are about what diagnostics are appropriate 
under the standard.  They are *not* about how code generation might allow 
for possible uses of certain source code constructs as if they were 
flexible array members.  The two contexts may very well require different 
notions of what counts as a flexible array member.

-- 
Joseph S. Myers
joseph@codesourcery.com