Re: ISO C3X proposal: nonnull qualifier

[Joseph Myers <joseph@codesourcery.com>]

On Mon, 15 Nov 2021, Alejandro Colomar (man-pages) via Gcc wrote:

> Hi Joseph,
> 
> On 11/15/21 23:17, Joseph Myers wrote:
> > On Mon, 15 Nov 2021, Alejandro Colomar (man-pages) via Gcc wrote:
> > 
> > > How is restrict handling that problem of lvalue-to-rvalue already?
> > 
> > restrict has tricky rules about "based on" (6.7.3.1).
> 
> Hmm, I think I can "base on" that,
> to define what I had in mind. :)

"based on" is about optimizations; I think it's even less suited to 
anything relating to diagnostics than it is to optimization.

To restrict assignment between different kinds of pointers, I'd think 
you'd want pointer type variants that differ in some way *other* than 
qualifiers, a way that's unaffected by lvalue-to-rvalue conversion, but 
that comes with its own rules on implicit conversion as if by assignment 
(6.5.16.1) (though then you also need to work out what's allowed in terms 
of mixing these pointer type variants in all the other operations allowing 
pointers, what type results of pointer arithmetic have, etc.).  And there 
should surely also be some way of converting a normal pointer to this 
variant with a runtime check for NULL.

Note that discussion of prior art in such a proposal should also consider 
relevant prior art (for constraining possible values of a variable through 
the type system) in C++ or other languages if possible.

-- 
Joseph S. Myers
joseph@codesourcery.com