[PATCH] D43576: Solution to fix PR27066 - Redefinition with same mangled name as another definition (dllexport and uuid)

[David Majnemer via Phabricator via cfe-commits]

majnemer added a comment.

In https://reviews.llvm.org/D43576#1016512, @rsmith wrote:

> Do we need to also track whether the argument is a pointer or reference to a UUID (and also the cv-qualifiers)? For the `Declaration` case, we track this by tracking the corresponding parameter type; the same thing would presumably work here.
>
> In https://reviews.llvm.org/D43576#1016295, @majnemer wrote:
>
> > We should really, really avoid making this sort of change without first trying to desugar uuidof into a reference to a variable. That would solve a ton of problems, problems like this one.
>
>
> This desugaring approach is not how we generally do things in Clang. The fact that MS exposes a variable that can be named from user code is, in my opinion, simply a bug in their implementation -- their implementation details are leaking -- and not part of the actual semantics here. I view this as exactly analogous to `typeid` (which would have exactly the same problems if its result could be used as a non-type template parameter); as with `typeid`, `__uuidof` notionally produces a global object not corresponding to any variable. If we want to model this as a declaration, we could add a new `Decl` subclass for these uuid objects (and eventually also for objects produced by `typeid`). But I don't think we should model them as variables unless that's actually part of their intended semantics.


Here's my thinking: the `__uuidof` expression literally declares a variable called `_GUID_ddb47a6a_0f23_11d5_9109_00e0296b75d3` of type `__s_GUID` which is why it behaves the way it does: https://godbolt.org/g/74FY7U

I don't think it is reasonable to invent new semantics which are different from the MSVC ones because we find the MSVC ones inelegant. The huge upside I see from matching their behavior is that the implementation is dramatically simpler, we have a considerable amount of mumbo jumbo in the template instantiation code to handle `__uuidof` correctly and it looks like we will need some more.

What is the relative upside to a new kind of Decl? Better AST fidelity?


https://reviews.llvm.org/D43576