[clang] Reapply "[Clang][Sema] Refactor collection of multi-level template argument lists (#106585)" (PR #111173)

[Krystian Stasiowski via cfe-commits]

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@@ -4159,7 +4159,7 @@ FunctionTemplateDecl *FunctionDecl::getPrimaryTemplate() const {
   if (FunctionTemplateSpecializationInfo *Info
         = TemplateOrSpecialization
             .dyn_cast<FunctionTemplateSpecializationInfo*>()) {
-    return Info->getTemplate();
+    return Info->getTemplate()->getMostRecentDecl();
----------------
sdkrystian wrote:

> Why do we need to get the most recent declaration in all these places?

@mizvekov One of the changes made by this patch is to set the `isMemberSpecialization` flag on a per-declaration basis. This ensures that `getTemplateInstantiationArgs` behaves correctly for implicitly instantiated declarations that are explicitly specialized for an implicitly instantiated specialization of their enclosing class template.

When we look up the name of a template, we typically find the most recent declaration of that template. If that declaration is an explicit specialization of a member template, we use the correct declaration when we try to find the definition of that template. However, if the member template is explicitly specialized _after_ we lookup its name, we end up with an expression naming the implicitly instantiated in-class declaration of that template. If we call `getTemplateInstantiationPattern` on that declaration, it will incorrectly return the pattern from the primary template.

https://github.com/llvm/llvm-project/pull/111173