[clang] [AST] Iterate redecls starting from the canonical one in getRawCommentsForAnyRedecl() (PR #108475)

[Younan Zhang via cfe-commits]

zyn0217 wrote:

So the problem is that we would have another implicitly-created explicit template specialization `foo<int, int>` in the process of the DeduceTemplateArguments, and that specialization would share the same `FunctionTemplateSpecializationInfo` with the eventual template specialization that ends up being `D` in the `getRawCommentForAnyRedecl`, while the implicitly-created specialization would

1) have the explicit specialization flag set in `CheckFunctionTemplateSpecialization`
2) become the previous declaration of the last explicit specialization
3) have no `ExtInfo` set so one cannot get any template parameters from it.

So the status-quo is seemingly violating the contracts of `NumTemplParamLists` / `TemplParamLists` in light of their intents.

The `TemplateParameterList` would be attached to the newly created explicit specialization prior to the call to `CheckFunctionTemplateSpecialization()`, so one solution (which essentially preserves the ExtInfo in the implicitly-created specialization) is to add the following to `CheckFunctionTemplateSpecialization(),` in the code block below the comment.

```cpp
// Mark the prior declaration as an explicit specialization, so that later
// clients know that this is an explicit specialization.
```

```cpp
SmallVector<TemplateParameterList *, 4> TPL;
for (unsigned I = 0; I < FD->getNumTemplateParameterLists(); ++I)
  TPL.push_back(FD->getTemplateParameterList(I));
Specialization->setTemplateParameterListsInfo(Context, TPL);
```

An alternative might be to teach `DeclInfo::fill()` not to rely on `getNumTemplateParameterLists()`, but that way we still lack the `TemplateParameterList`.



https://github.com/llvm/llvm-project/pull/108475