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

[via cfe-commits]

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@@ -951,28 +959,124 @@ Sema::ActOnDecompositionDeclarator(Scope *S, Declarator &D,
   return New;
 }
 
+// CheckBindingsCount
+//  - Checks the arity of the structured bindings
+//  - Creates the resolved pack expr if there is
+//    one
+static bool CheckBindingsCount(Sema &S, DecompositionDecl *DD,
+                               QualType DecompType,
+                               ArrayRef<BindingDecl *> Bindings,
+                               unsigned MemberCount) {
+  auto BindingWithPackItr =
+      std::find_if(Bindings.begin(), Bindings.end(),
+                   [](BindingDecl *D) -> bool { return D->isParameterPack(); });
+  bool HasPack = BindingWithPackItr != Bindings.end();
+  bool IsValid;
+  if (!HasPack) {
+    IsValid = Bindings.size() == MemberCount;
+  } else {
+    // there may not be more members than non-pack bindings
+    IsValid = MemberCount >= Bindings.size() - 1;
+  }
+
+  if (IsValid && HasPack) {
+    // create the pack expr and assign it to the binding
+    unsigned PackSize = MemberCount - Bindings.size() + 1;
+    QualType PackType = S.Context.getPackExpansionType(
+        S.Context.DependentTy, std::nullopt, /*ExpectsPackInType=*/false);
+    BindingDecl *BD = (*BindingWithPackItr);
+    BD->setBinding(PackType,
+                   ResolvedUnexpandedPackExpr::Create(
+                       S.Context, DD->getBeginLoc(), DecompType, PackSize));
----------------
cor3ntin wrote:

can we do at least do something like

```cpp
auto* DD =  cast<DecompositionDecl>(Src);
 if (CheckBindingsCount(S, DD, DecompType, Bindings,
                         NumElems))
    return true;

 if(ResolvePackInDecomposition(DD, /*TotalSize=*/NumElems)) // create the nested bindings but does not call `setBindings`
    return true;

for (auto *BD : DD->flat_bindings()) {
  /*...*/
 BD->setBinding( /*...*/)
}

```

IE, my question is can we reuse `flat_bindings()` and not have `BindingInitWalker`

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