[clang] Qualify non-dependent types of a class template with its declaration (PR #67566)

[Luca Di sera via cfe-commits]

diseraluca wrote:

> @diseraluca, thanks for the thorough description. The point of these routines is to produce code that compiles. I am not sure if we change `Foo<int>::Bar` with `Foo<T>::Bar` it will compile.
> 
> > Due to the way the current codebase is set up, the chosen specialization is `QFuture<void>` so that, for example, `QFuture::constBegin()` would be shown to the user as returning a `QFuture<void>::const_iterator`. Nonetheless, `QFuture<void>` is the only specialization that cannot have a `const_iterator` and, as a consequence, doesn't have a `constBegin` method in the first place.
> 
> I suspect that `QFuture<void>` is an explicit specialization. The intent of the code was to pick up an implicit specialization which follows much more closely the template pattern. Would selecting the first non-explicit instantiation fix your usecase?

Thank you for your answer @vgvassilev.

I gave it a quick try, and we would still end up with the same result in our codebase. But, generally, this would not probably be feasible for us as a solution.

Since that is the case, and we have somewhat conflicting requirements, I think the best path would be for me to indefinitely maintain a custom implementation of `getFullyQualified*`, leaving upstream as is.

I would thus suggest to close this without merging.

https://github.com/llvm/llvm-project/pull/67566