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

[Jason Rice via cfe-commits]

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@@ -757,23 +775,40 @@ bool Sema::CheckParameterPacksForExpansion(
   bool HaveFirstPack = false;
   std::optional<unsigned> NumPartialExpansions;
   SourceLocation PartiallySubstitutedPackLoc;
+  typedef LocalInstantiationScope::DeclArgumentPack DeclArgumentPack;
 
   for (UnexpandedParameterPack ParmPack : Unexpanded) {
     // Compute the depth and index for this parameter pack.
     unsigned Depth = 0, Index = 0;
     IdentifierInfo *Name;
     bool IsVarDeclPack = false;
+    ResolvedUnexpandedPackExpr *ResolvedPack = nullptr;
 
     if (const TemplateTypeParmType *TTP =
             ParmPack.first.dyn_cast<const TemplateTypeParmType *>()) {
       Depth = TTP->getDepth();
       Index = TTP->getIndex();
       Name = TTP->getIdentifier();
+    } else if (auto *RP =
+                   ParmPack.first.dyn_cast<ResolvedUnexpandedPackExpr *>()) {
+      ResolvedPack = RP;
     } else {
       NamedDecl *ND = cast<NamedDecl *>(ParmPack.first);
       if (isa<VarDecl>(ND))
         IsVarDeclPack = true;
-      else
+      else if (isa<BindingDecl>(ND)) {
+        // Find the instantiated BindingDecl and check it for a resolved pack.
+        llvm::PointerUnion<Decl *, DeclArgumentPack *> *Instantiation =
+            CurrentInstantiationScope->findInstantiationOf(ND);
+        Decl *B = cast<Decl *>(*Instantiation);
+        Expr *BindingExpr = cast<BindingDecl>(B)->getBinding();
+        ResolvedPack =
+            dyn_cast_if_present<ResolvedUnexpandedPackExpr>(BindingExpr);
+        if (!ResolvedPack) {
+          ShouldExpand = false;
+          continue;
+        }
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ricejasonf wrote:

This case is definitely reached. (I tried it.) I think I could make this `cast_if_present`.

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