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Re: [PATCH] Add a new type attribute always_alias (PR79671)
On Fri, 7 Apr 2017, Florian Weimer wrote:
> On 04/06/2017 09:16 PM, Richard Biener wrote:
> > On April 6, 2017 8:12:29 PM GMT+02:00, Bernd Edlinger
> > <bernd.edlinger@hotmail.de> wrote:
> > > But isn't the effective type changed by the assignment b[1] = 0;
> > > as described in 6.5(6):
> > > "If a value is stored into an object having no declared type through an
> > > lvalue having a type that is not a character type, then the type of the
> > > lvalue becomes the effective type of the object for that access and for
> > > subsequent accesses that do not modify the stored value."
> >
> > Yes. I think the example is valid. At least GCCs memory model makes it so.
>
> As far as I understand the standard, C does not permit changing the effective
> type of an object if it has a declared type (at least not without a union).
> If GCC supports it, that's an undocumented GCC extension.
The GCC middle-end supports it because C++ supports it and there is no
way for the C FE to tell the middle-end that this is not valid.
Richard.