Mordante added inline comments.
================
Comment at: clang/include/clang/Basic/AttrDocs.td:5273
+ namespace empty_namespace {};
+ using empty_namespace::does_not_exist __attribute__((using_if_exists)); //
no error!
+
----------------
erik.pilkington wrote:
> aaron.ballman wrote:
> > erik.pilkington wrote:
> > > Mordante wrote:
> > > > Mordante wrote:
> > > > > Can you add an example using `[[clang::using_if_exists]]` or use that
> > > > > instead of this attribute?
> > > > Why is the attribute placed here? I would expect the attribute before
> > > > the declaration `[[clang::using_if_exists]] using
> > > > empty_namespace::does_not_exist;`
> > > The attribute is written like that because clang rejects C++ style
> > > attributes on using declarations, and only accepts attributes in that
> > > position. I think this is the first attribute we actually support on
> > > using-declarations, so maybe we should consider supporting it.
> > > I think this is the first attribute we actually support on
> > > using-declarations, so maybe we should consider supporting it.
> >
> > This isn't the first attribute we *should* be supporting on using
> > declarations. Any attribute that appertains to a `NamedDecl` should apply
> > as should the annotate attribute.
> >
> > However:
> > ```
> > [[clang::whatever]] using foo::bar; // Correct to reject, #1
> > using foo::bar [[clang::whatever]]; // Correct to reject, #2
> > ```
> > #1 is rejected because it's a declaration-statement and those cannot have a
> > leading attribute-specifier-seq
> > (http://eel.is/c++draft/stmt.stmt#stmt.pre-1). #2 is rejected because the
> > using-declaration cannot have a trailing attribute-specifier-seq
> > (http://eel.is/c++draft/namespace.udecl#nt:using-declaration).
> >
> > This seems like a case where we may want to explore an extension to C++
> > that we propose to WG21.
> > This isn't the first attribute we *should* be supporting on using
> > declarations. Any attribute that appertains to a NamedDecl should apply as
> > should the annotate attribute.
>
> Yeah, agreed. Its just that Sema was failing to call ProcessDeclAttributeList
> for UsingDecls, so no attributes were actually getting applied in practice.
>
> Okay, if our policy is to only parse C++-style attributes in places that the
> C++ grammar allows them then I agree that this would be an issue for the
> standards committee.
I would be in favour to implement this as an extension and propose it to the
committee. I wonder whether it's not allowed because the committee objected or
nobody wrote a paper. (Attributes are already allowed on using namespace
directives http://eel.is/c++draft/namespace.udir.)
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D90188/new/
https://reviews.llvm.org/D90188
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