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

[Richard Smith via cfe-commits]

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@@ -4213,8 +4226,35 @@ class DecompositionDecl final
   static DecompositionDecl *CreateDeserialized(ASTContext &C, GlobalDeclID ID,
                                                unsigned NumBindings);
 
-  ArrayRef<BindingDecl *> bindings() const {
-    return llvm::ArrayRef(getTrailingObjects<BindingDecl *>(), NumBindings);
+  // Provide the range of bindings which may have a nested pack.
+  llvm::ArrayRef<BindingDecl *> bindings() const {
+    return {getTrailingObjects<BindingDecl *>(), NumBindings};
+  }
+
+  // Provide a flattened range to visit each binding.
+  auto flat_bindings() const {
+    llvm::ArrayRef<BindingDecl *> Bindings = bindings();
+    llvm::ArrayRef<Expr *> PackExprs;
+
+    // Split the bindings into subranges split by the pack.
+    auto S1 = Bindings.take_until(
+        [](BindingDecl *BD) { return BD->isParameterPack(); });
+
+    Bindings = Bindings.drop_front(S1.size());
+    if (!Bindings.empty()) {
+      PackExprs = Bindings.front()->getBindingPackExprs();
+      Bindings = Bindings.drop_front();
+    }
+
+    auto S2 = llvm::map_range(PackExprs, [](Expr *E) {
+      auto *DRE = cast<DeclRefExpr>(E);
+      return cast<BindingDecl>(DRE->getDecl());
+    });
+
+    // llvm::concat must take temporaries or it will capture
+    // references.
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zygoloid wrote:

I found this comment a bit confusing, because `move` produces a reference rather than a temporary. How about something like "Pass xvalues to llvm::concat to request that it makes a copy."

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