[cfe-dev] Unneeded declarations invalidation

Douglas Gregor dgregor at apple.com
Tue Jul 21 07:15:42 PDT 2009

On Jul 21, 2009, at 1:05 AM, Abramo Bagnara wrote:

> Douglas Gregor ha scritto:
>> The real problem here is that we're replacing the type of the field
>> with 'int', when there's no reason to do that. However, I feel that  
>> it
>> is important to mark this declaration as invalid, because it *is*
>> invalid. So, I've committed the change not to replace the type with
>> 'int', but the declaration is still marked invalid:
>> 	http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090720/019240.html
> The problem with this choice is that the AST is needlessly mutilated.
> Take this example:
> void f(int n) {
>  struct s {
>    int i;
>    int a[n];
>  };
>  struct s a, b;
>  a.i = 3;
>  a.a[0] = 2;
> }
> With your changes the last statement of the function is not inserted  
> in
> the AST.
> Which drawbacks you see to not invalidate the declaration and then to
> permit the building of a complete AST nevertheless, once taken for
> granted that the code generation phase will never be run?

It breaks the invariant that there are no variable-length structures,  
which means that we can't compute sizeof(s). Since the well-formedness  
(and, in C++, the AST itself) can depend on the value of sizeof(s),  
later semantic analysis is likely to fail. We mark invalid  
declarations as invalid to avoid such cascading failures.

	- Doug

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