[clang] [Serialization] Fix lazy template loading (PR #133057)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > > Unfortunately, I don't have an easy idea to fix it - going back to the old code and just calling `getMostRecentDecl` at the beginning doesn't pass the `Modules/GH170084.cpp` test case...
> > 
> > 
> > I feel it may not be too hard to implement a `noload_redecls()`. You can get the most recent decl without calling `getMostRecentDecl ()` (that will trigger the deserialization of all redecls, I hate it) in Redeclarable.h internally. And you can simply traverse the redecls.
> 
> As I hinted above, that doesn't work. More specifically, I tried
> 
> ```diff
> diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
> index 882d54f31280..d63824c4ba18 100644
> --- a/clang/lib/Serialization/ASTReaderDecl.cpp
> +++ b/clang/lib/Serialization/ASTReaderDecl.cpp
> @@ -2107,8 +2107,9 @@ void ASTDeclMerger::MergeDefinitionData(
>      auto *Def = DD.Definition;
>      DD = std::move(MergeDD);
>      DD.Definition = Def;
> -    for (auto *D : Def->redecls())
> -      cast<CXXRecordDecl>(D)->DefinitionData = ⅅ
> +    Def = Def->getMostRecentDecl();
> +    while ((Def = Def->getPreviousDecl()))
> +      cast<CXXRecordDecl>(Def)->DefinitionData = ⅅ
>      return;
>    }
>  
> ```

You can't use `getMostRecentDecl `, this will trigger deserializations. Then it is the same with before. You need to implement `noload_redecls()`, possibly in Redeclarable.h

https://github.com/llvm/llvm-project/pull/133057