From 0016f63547f70f969eeb6030d8d7433b05cae934 Mon Sep 17 00:00:00 2001
From: Daniel Bertalan <dani@danielbertalan.dev>
Date: Tue, 28 Mar 2023 14:07:25 +0200
Subject: [PATCH] AK: Fix Clang 16 UBSan issue with zero-length `Array`

The current implementation of `Array<T, 0>` has a zero-length C array as
its storage type. While this is accepted as a GNU extension, when
compiling with Clang 16, an UBSan error is raised every time an object
is accessed whose only field is a zero-length array.

This is likely a bug in Clang 16's implementation of UBSan, which has
been reported here: https://github.com/llvm/llvm-project/issues/61775
---
 AK/Array.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/AK/Array.h b/AK/Array.h
index 26c0a225b50..3bb11998a37 100644
--- a/AK/Array.h
+++ b/AK/Array.h
@@ -13,6 +13,16 @@
 
 namespace AK {
 
+namespace Detail {
+// This type serves as the storage of 0-sized `AK::Array`s. While zero-length `T[0]`
+// is accepted as a GNU extension, it causes problems with UBSan in Clang 16.
+template<typename T>
+struct EmptyArrayStorage {
+    T& operator[](size_t) const { VERIFY_NOT_REACHED(); }
+    constexpr operator T*() const { return nullptr; }
+};
+}
+
 template<typename T, size_t Size>
 struct Array {
     using ValueType = T;
@@ -109,7 +119,7 @@ struct Array {
         return value;
     }
 
-    T __data[Size];
+    Conditional<Size == 0, Detail::EmptyArrayStorage<T>, T[Size]> __data;
 };
 
 template<typename T, typename... Types>