From 3d3c7bbafb2cf2e827dcce3ff4eff02dbb67b582 Mon Sep 17 00:00:00 2001
From: "John F. Carr" <jfc@mit.edu>
Date: Sat, 9 Dec 2023 16:35:02 -0500
Subject: [PATCH] If a HyperobjectType has an incomplete element type it must
 be marked as containing an error.

---
 clang/lib/AST/Type.cpp      |  2 ++
 clang/lib/Sema/SemaType.cpp | 16 +++++++++-------
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/clang/lib/AST/Type.cpp b/clang/lib/AST/Type.cpp
index 5b7463d8d403..d32e47372e40 100644
--- a/clang/lib/AST/Type.cpp
+++ b/clang/lib/AST/Type.cpp
@@ -3206,6 +3206,8 @@ HyperobjectType::HyperobjectType(QualType Element, QualType CanonicalPtr,
   : Type(Hyperobject, CanonicalPtr, Element->getDependence()),
     ElementType(Element), Identity(i), Reduce(r),
     IdentityID(ifn), ReduceID(rfn) {
+  if (Element->isIncompleteType()) // diagnosed in caller
+    addDependence(TypeDependence::Error);
 }
 
 bool HyperobjectType::hasCallbacks() const {
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 0cff893f0c0d..e4b9c77abacf 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2442,20 +2442,22 @@ Expr *Sema::ValidateReducerCallback(Expr *E, unsigned NumArgs,
 
 QualType Sema::BuildHyperobjectType(QualType Element, Expr *Identity,
                                     Expr *Reduce, SourceLocation Loc) {
-  QualType Result = Element;
+  // This function must return a HyperobjectType with the given
+  // element type, otherwise the rest of the front end will get angry.
+  // Template instantiation is quite strict about preserving structure.
+  // The compiler will also get angry if the element type is incomplete
+  // and the HyperobjectType is not marked as containing an error.
   if (!RequireCompleteType(Loc, Element, CompleteTypeKind::Normal,
                            diag::incomplete_hyperobject)) {
-    if (std::optional<unsigned> Code = ContainsHyperobject(Result))
-      Diag(Loc, *Code) << Result;
+    if (std::optional<unsigned> Code = ContainsHyperobject(Element))
+      Diag(Loc, *Code) << Element;
   }
 
   Identity = ValidateReducerCallback(Identity, 1, Loc);
   Reduce = ValidateReducerCallback(Reduce, 2, Loc);
 
-  // The result of this function must be HyperobjectType if it is called
-  // from C++ template instantiation when rebuilding an existing hyperobject
-  // type.
-  return Context.getHyperobjectType(Result, Identity, Reduce);
+  // The result will be marked erroneous if Element is incomplete.
+  return Context.getHyperobjectType(Element, Identity, Reduce);
 }
 
 /// Build a Read-only Pipe type.