Re: [PATCH 1/2] Handle component_ref to a structre/union field including flexible array member [PR101832]

[Joseph Myers <joseph@codesourcery.com>]

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

> Then, this routine (flexible_array_type_p) is mainly for diagnostic purpose.
> It cannot be used to determine whether the structure/union type recursively
> include a flexible array member at the end.
> 
> Is my understanding correct?

My comments were about basic principles of what gets diagnosed, and the 
need for different predicates in different contexts; I wasn't trying to 
assert anything about how that maps onto what functions should be used in 
what contexts.

> >> 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".
> 
> With the -fstrict-flex-arrays available, we should be able to diagnose
> the flexible array member per gnu extension (i.e [0] or [1]) the same as []. 

There are different sorts of diagnostic that might be involved.

* Simply having [0] or [1] at the end of a structure embedded in another 
structure isn't appropriate to diagnose, because [0] and [1] have 
perfectly good meanings in such a context that aren't trying to be 
flexible array members at all.  [0] might be an empty type (possibly one 
that wouldn't be empty when built with a different configuration).  [1] 
might be the use of arrays in C to produce a passed-by-reference type.

* Trying to use such an embedded [0] or [1] array as if it were a flexible 
array member - i.e. accessing any member of the [0] array, or any member 
other than the [0] member of the [1] array - *is* a sign of the 
problematic use as a flexible array member, that might be appropriate to 
diagnose.  (Actually I'd guess the array index tends to be non-constant in 
accesses, and it would be odd to use a non-constant index when you mean 
that constant always to be 0, which it would need to be in the 
non-flexible case.)

-- 
Joseph S. Myers
joseph@codesourcery.com