AK: Accomodate always-32-bit data member pointers in IntrusiveList

This only exists on windows, but we've made an effort to keep jakt
working on windows, so let's support this silliness.

1 files changed, 8 insertions, 1 deletions

diff --git a/AK/IntrusiveList.h b/AK/IntrusiveList.h
index 9475efd492..b671a7a11c 100644
--- a/AK/IntrusiveList.h
+++ b/AK/IntrusiveList.h
@@ -375,13 +375,20 @@ inline typename IntrusiveList<T, Container, member>::ConstIterator IntrusiveList
 template<class T, typename Container, SubstitutedIntrusiveListNode<T, Container> T::*member>
 inline T* IntrusiveList<T, Container, member>::node_to_value(SubstitutedIntrusiveListNode<T, Container>& node)
 {
+    // Note: A data member pointer is a 32-bit offset in the Windows ABI (both x86 and x86_64),
+    //       whereas it is an appropriately sized ptrdiff_t in the Itanium ABI, the following ensures
+    //       that we always use the correct type for the subtraction.
+    using EquivalentNumericTypeForDataMemberPointer = Conditional<sizeof(member) == sizeof(ptrdiff_t), ptrdiff_t, u32>;
+    static_assert(sizeof(EquivalentNumericTypeForDataMemberPointer) == sizeof(member),
+        "The equivalent numeric type for the data member pointer must have the same size as the data member pointer itself.");
+
     // Note: Since this might seem odd, here's an explanation on what this function actually does:
     //       `node` is a reference that resides in some part of the actual value (of type T), the
     //       placement (i.e. offset) of which is described by the pointer-to-data-member parameter
     //       named `member`.
     //       This function effectively takes in the address of the data member, and returns the address
     //       of the value (of type T) holding that member.
-    return bit_cast<T*>(bit_cast<unsigned char*>(&node) - bit_cast<unsigned char*>(member));
+    return bit_cast<T*>(bit_cast<unsigned char*>(&node) - bit_cast<EquivalentNumericTypeForDataMemberPointer>(member));
 }
 
 template<typename T, typename Container>