[cfe-dev] Implementing P1099R5 (using enum) in clang

[David Rector via cfe-dev]

You definitely want to represent the original declaration in the AST — never just expand it right away into the underlying semantics.

Others know the AST much better than I do, but: I think you could *also* implement the easy way as well, by adding implicit UsingShadowDecls for each enumerator, each linked to the original UsingEnumDecl.  Or better yet, somehow add a single shadow decl that is a transparent lookup context, somehow.

I believe UsingShadowDecls are introduced by UsingDecls to handle whenever multiple individual declarations need to be introduced into a context — someone correct me if I’m wrong.  There is definitely precedent for introducing implicit declarations simply for lookup purposes, e.g. VarTemplateDecl instantiations get added to a DeclContext.  It’s annoying to deal with but since it’s already being done, I see no reason not to introducing UsingShadowDecls to handle the implementation in this way.

- Dave

> On Jul 10, 2020, at 7:01 AM, Shachaf Co via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Hi all,
> 
> I'm new to Clang, and decided that I want to implement a small C++20 feature - the "using enum" proposal (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1099r5.html <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1099r5.html>).
> 
> The first part of the proposal, supporting syntax such as "using Color::RED" is easy enough to implement, and basically requires removing an if-statement and adding some diagnostics.
> 
> However, I struggle finding the correct way to implement the "using enum Color" syntax. There are two approaches that I see when trying to implement it.
> 
> The first approach is "expanding" it to a list of using-declarations (i.e "using enum Color" would be equivalent to "using Color::RED, Color::BLUE, ...). This approach is easier to implement, however the resulting AST won't be as accurate.
> 
> The second approach is creating a new AST node representing a "UsingEnumDecl". This will generate a more "correct" AST, however it is much more complicated to implement - it is more similar to using directives ("using namespace ns"), whose implementation is complicated and heavily affects the name lookup.
> 
> What do you think is the correct way to implement the feature? Is reducing the complexity worth degrading the accuracy of the AST?
> 
> Thanks,
> Shachaf
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