[cfe-dev] Determining if a UsingDecl/UsingShadowDecl was used in the instantiation of a variable.

[Eric Bayer via cfe-dev]

Thanks for the answer.  Damn.  Yeah, that is the conclusion I was coming 
to as well.  We had a similar problem capturing pre-processor macros as 
they really are not part of the AST. Eventually I figured out a way to 
do it because I could capture data during the expansions and maintain a 
data structure on the side.  Unfortunately for name resolution there 
seems to be no similar callback/hook.

I also had wondered if there was some stage when maybe they'd be 
unresolved and I could build a data structure for later reference, but 
I've still not explored that avenue.

    -Eric

On 10/28/2019 1:08 PM, David Blaikie wrote:
> I don't believe that information is preserved in Clang's AST (if it 
> was, we'd probably want to use it from the DWARF emission (GCC 
> judiciously emits only referenced alias directives, whereas clang just 
> emits them all, unfortunately) - and also to power a -Wunused... hmm, 
> I thought GCC had a warning for unused using directives, but I can't 
> seem to reproduce that now, so maybe I hallucinated).
>
> On Tue, Oct 22, 2019 at 2:02 PM Eric Bayer via cfe-dev 
> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>
>     Hi,
>
>     I have a plugin I'm working on a clang plugin where we need to
>     determine all the things used in the creation of a piece of code. 
>     Right now we have a situation as follows:
>
>     namespace X { class Y { enum Z { A=0, B, C }; }; };
>     using X::Y;
>
>     void MyFunc() {
>     	Y::Z q;
>              ...
>     }
>
>     What we currently do is traverse the whole of MyFunc looking for
>     referenced elements using VisitStmt().  The problem I have is that
>     I can't find any way to determine that "using X::Y;" was used and
>     that it was involved in resolving "Y::Z" to "X::Y::Z" in the
>     creation of "q". This has not been a problem for all sorts of
>     sugar (like typedefs, type aliases, etc), but I am beside myself
>     to find a way in this case.  Is there any way to do this? I'm even
>     interested if it's a sort of "long way around" solution.  It's
>     surprising to me that this has been this hard... there have been
>     breadcrumbs for every other piece of syntactic sugar.
>
>     Thanks in advance,
>
>         -Eric
>
>     PS - I thought I had found some clues, but they only seemed to
>     work for functions in one case (DeclRefExpr), and templates in
>     another (getInstantiatedFromUsingDecl()).
>
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