[llvm-branch-commits] [clang] [Clang] [C++26] Expansion Statements (Part 6: Destructuring Expansion Statements) (PR #169685)

[Corentin Jabot via llvm-branch-commits]

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@@ -384,8 +429,59 @@ StmtResult Sema::BuildNonEnumeratingCXXExpansionStmtPattern(
         Data.IterDecl, LParenLoc, ColonLoc, RParenLoc);
   }
 
-  Diag(ESD->getLocation(), diag::err_expansion_statements_todo);
-  return StmtError();
+  // If not, try destructuring.
+  StmtResult DecompDeclStmt = BuildDestructuringDecompositionDecl(
+      *this, ExpansionInitializer, ColonLoc, ExpansionVar->isConstexpr(),
+      LifetimeExtendTemps);
+  if (DecompDeclStmt.isInvalid()) {
+    ActOnInitializerError(ExpansionVar);
+    return StmtError();
+  }
+
+  auto *DS = DecompDeclStmt.getAs<DeclStmt>();
+  auto *DD = cast<DecompositionDecl>(DS->getSingleDecl());
+  if (DD->isInvalidDecl())
+    return StmtError();
+
+  // Synthesise an InitListExpr to store the bindings; this essentially lets us
+  // desugar the expansion of a destructuring expansion statement to that of an
+  // enumerating expansion statement.
+  SmallVector<Expr *> Bindings;
+  for (BindingDecl *BD : DD->bindings()) {
+    Expr *Element = BuildDeclRefExpr(BD, BD->getType().getNonReferenceType(),
+                                     VK_LValue, ColonLoc);
+
+    // CWG 3149: If the expansion-initializer is an lvalue, then vi is ui;
+    // otherwise, vi is static_cast<decltype(ui)&&>(ui).
+    if (!ExpansionInitializer->isLValue()) {
+      QualType Ty =
+          BuildReferenceType(getDecltypeForExpr(Element), /*LValueRef=*/false,
+                             ColonLoc, /*Entity=*/DeclarationName());
+      TypeSourceInfo *TSI = Context.getTrivialTypeSourceInfo(Ty);
+      ExprResult Cast = BuildCXXNamedCast(
+          ColonLoc, tok::kw_static_cast, TSI, Element,
+          SourceRange(ColonLoc, ColonLoc), SourceRange(ColonLoc, ColonLoc));
+      assert(!Cast.isInvalid() && "cast to rvalue reference type failed?");
+      Element = Cast.get();
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cor3ntin wrote:

Do we actually have to do all that, or can we create `BuildDeclRefExpr` with an rvalue?

https://github.com/llvm/llvm-project/pull/169685