[clang] [Clang][P1061] Add stuctured binding packs (PR #121417)

[Younan Zhang via cfe-commits]

================
@@ -1166,26 +1166,54 @@ TemplateDeclInstantiator::VisitTypeAliasTemplateDecl(TypeAliasTemplateDecl *D) {
 
 Decl *TemplateDeclInstantiator::VisitBindingDecl(BindingDecl *D) {
   auto *NewBD = BindingDecl::Create(SemaRef.Context, Owner, D->getLocation(),
-                                    D->getIdentifier());
+                                    D->getIdentifier(), D->getType());
   NewBD->setReferenced(D->isReferenced());
   SemaRef.CurrentInstantiationScope->InstantiatedLocal(D, NewBD);
+
   return NewBD;
 }
 
 Decl *TemplateDeclInstantiator::VisitDecompositionDecl(DecompositionDecl *D) {
   // Transform the bindings first.
+  // The transformed DD will have all of the concrete BindingDecls.
   SmallVector<BindingDecl*, 16> NewBindings;
-  for (auto *OldBD : D->bindings())
+  ResolvedUnexpandedPackExpr *OldResolvedPack = nullptr;
+  for (auto *OldBD : D->bindings()) {
+    Expr *BindingExpr = OldBD->getBinding();
+    if (auto *RP = dyn_cast_if_present<ResolvedUnexpandedPackExpr>(BindingExpr))
+      OldResolvedPack = RP;
     NewBindings.push_back(cast<BindingDecl>(VisitBindingDecl(OldBD)));
+  }
   ArrayRef<BindingDecl*> NewBindingArray = NewBindings;
 
-  auto *NewDD = cast_or_null<DecompositionDecl>(
+  auto *NewDD = cast_if_present<DecompositionDecl>(
       VisitVarDecl(D, /*InstantiatingVarTemplate=*/false, &NewBindingArray));
 
   if (!NewDD || NewDD->isInvalidDecl())
     for (auto *NewBD : NewBindings)
       NewBD->setInvalidDecl();
 
+  if (OldResolvedPack) {
+    // Mark the holding vars (if any) in the pack as instantiated since
+    // they are created implicitly.
+    auto Bindings = NewDD->bindings();
+    auto BPack = std::find_if(
+        Bindings.begin(), Bindings.end(),
+        [](BindingDecl *D) -> bool { return D->isParameterPack(); });
----------------
zyn0217 wrote:

```suggestion
    auto BPack = llvm::find_if(
        Bindings, [](BindingDecl *D) { return D->isParameterPack(); });
```

https://github.com/llvm/llvm-project/pull/121417