AK: Add abstraction for forcing empty base optimization on Windows

Without this annotation, the MSVC ABI is reluctant to apply EBO even in cases that are basically guaranteed on the Itanium ABI by modern compilers. This fixes an UBSAN issue with Variant on Windows.
Author: https://github.com/ayeteadoe Commit: 3fd05306fc Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4962 Reviewed-by: https://github.com/ADKaster ✅ Reviewed-by: https://github.com/Hendiadyoin1 Reviewed-by: https://github.com/R-Goc
2025-08-20 01:11:07 +00:00 · 2025-06-16 15:08:50 -06:00 · 2025-06-16 15:08:50 -06:00 · 3fd05306fc · 2025-06-17 21:35:02 +00:00
commit 3fd05306fc
parent 75b0e9a199
3 changed files with 10 additions and 3 deletions
--- a/AK/Variant.h
+++ b/AK/Variant.h
@ -478,7 +478,7 @@ private:
    }

    template<typename... Fs>
-    struct Visitor : Fs... {
+    struct AK_COMPACT_EMPTY_BASES Visitor : Fs... {
        using Types = TypeList<Fs...>;

        Visitor(Fs&&... args)