[clang] [clang][bytecode] Fix placement new on multidimensional array elements (PR #191766)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/191766

The direct base of those pointers is not a union, i.e. `getRecord()` returns `nullptr`.

>From 54a462d53af7e21ff613a15f6eb10ee80f3a7b77 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Mon, 13 Apr 2026 09:18:48 +0200
Subject: [PATCH] adsf

---
 clang/lib/AST/ByteCode/Interp.cpp         |  9 +++++++--
 clang/test/AST/ByteCode/placement-new.cpp | 15 +++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/clang/lib/AST/ByteCode/Interp.cpp b/clang/lib/AST/ByteCode/Interp.cpp
index 3d75d5c857cf1..f4d6ef4dc8bd6 100644
--- a/clang/lib/AST/ByteCode/Interp.cpp
+++ b/clang/lib/AST/ByteCode/Interp.cpp
@@ -1990,7 +1990,12 @@ bool CheckNewTypeMismatch(InterpState &S, CodePtr OpPC, const Expr *E,
                           std::optional<uint64_t> ArraySize) {
   const Pointer &Ptr = S.Stk.peek<Pointer>();
 
-  if (Ptr.inUnion() && Ptr.getBase().getRecord()->isUnion())
+  auto directBaseIsUnion = [](const Pointer &Ptr) -> bool {
+    const Record *R = Ptr.getBase().getRecord();
+    return R && R->isUnion();
+  };
+
+  if (Ptr.inUnion() && directBaseIsUnion(Ptr))
     Ptr.activate();
 
   if (Ptr.isZero()) {
@@ -2071,7 +2076,7 @@ bool CheckNewTypeMismatch(InterpState &S, CodePtr OpPC, const Expr *E,
   }
 
   // Can't activate fields in a union, unless the direct base is the union.
-  if (Ptr.inUnion() && !Ptr.isActive() && !Ptr.getBase().getRecord()->isUnion())
+  if (Ptr.inUnion() && !Ptr.isActive() && !directBaseIsUnion(Ptr))
     return CheckActive(S, OpPC, Ptr, AK_Construct);
 
   return true;
diff --git a/clang/test/AST/ByteCode/placement-new.cpp b/clang/test/AST/ByteCode/placement-new.cpp
index 2c302ef7a8e7b..6091ab5602121 100644
--- a/clang/test/AST/ByteCode/placement-new.cpp
+++ b/clang/test/AST/ByteCode/placement-new.cpp
@@ -524,3 +524,18 @@ static_assert(intDestArray() == 0); // both-error {{not an integral constant exp
 
 constexpr void invalidDest() { new (undefinedfunction()) int; } // both-error {{use of undeclared identifier 'undefinedfunction'}}
 static_assert((invalidDest(), true)); // both-error {{not an integral constant expression}}
+
+namespace DirectBaseHasNoRecord {
+  constexpr int test_multidim_single_start() {
+    struct S {
+      union {
+        int storage[2][3];
+      };
+    };
+    S s;
+    new (&s.storage[0][0]) int(1); // both-note {{construction of subobject of member 'storage' of union with no active member is not allowed in a constant expression}}
+    return 13;
+  }
+  static_assert(test_multidim_single_start() == 13); // both-error {{not an integral constant expression}} \
+                                                     // both-note {{in call to}}
+}