[PATCH] Add gnu::diagnose_as attribute

[Matthias Kretz]

On Tuesday, 22 June 2021 22:12:42 CEST Jason Merrill wrote:
> On 6/22/21 4:01 PM, Matthias Kretz wrote:
> > On Tuesday, 22 June 2021 21:52:16 CEST Jason Merrill wrote:
> >> For alias templates, you probably want the attribute only on the
> >> templated class, not on the instantiations.
> > 
> > Oh good point. My current patch does not allow the attribute on alias
> > templates. Consider:
> > 
> > template <class T, class U>
> > 
> >    struct X {};
> > 
> > template <class T>
> > 
> >    using foo [[gnu::diagnose_as]] = X<T, int>;
> > 
> > I have no idea how this could work. I would have to set the attribute for
> > an implicit partial specialization (not that I know of the existence of
> > such a thing)? I.e. X<int, int> would have to be diagnosed as foo<int>,
> > but X<int, float> would have to be diagnosed as X<int, float>, not foo.
> > 
> > So if anything it should only support alias templates if they are strictly
> > "renaming" the type. I.e. their template parameters must match up exactly.
> > Can I constrain the attribute like this?
> 
> Yes.  You can check that with get_underlying_template.
> 
> Or you could support the above by putting the attribute on the
> instantiation with the TEMPLATE_INFO for foo<A> rather than a simple name.

Question, given:

  template <class T> using foo = bar<T>;

The diagnose_as attribute handler isn't called until e.g. `foo<int>` is 
instantiated. Meaning that even after the declaration of the alias template 
`bar<int>` will not be diagnosed as `foo<int>`, which happens only after the 
first use of `foo<int>`. I find that more confusing than helpful, even if the 
expectation would be that users only use the alias template.

So do you still expect alias templates to support diagnose_as? And if yes, how 
could I handle the attribute so that the diagnose_as attribute is applied to 
`bar` on declaration of `foo`?

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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
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