Giant loop back as this was extremely confusing to me and I just got around to investigating.

> On Jun 2, 2025, at 4:23 PM, Frederick Virchanza Gotham via Std-Proposals <std-proposals_at_[hidden]> wrote:
>
>
> My code doesn't cause address sanitizer to throw up an alarm. If you
> can beat address sanitizer then you can beat anything. Here's how I
> build my programs on Linux to debug them:
>
> set(ENV{ASAN_OPTIONS} "detect_invalid_pointer_pairs=2")
> set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -ggdb3
> -pedantic -Wall -Wextra -rdynamic -funwind-tables
> -fno-omit-frame-pointer -fno-common -pthread
> -fsanitize=address,leak,undefined,pointer-compare,pointer-subtract,float-divide-by-zero,float-cast-overflow
> -fsanitize-address-use-after-scope -fcf-protection=full
> -fstack-protector-all -fstack-clash-protection -fvtv-debug
> -fvtv-counts -finstrument-functions -D_GLIBC_DEBUG
> -D_GLIBC_DEBUG_PEDANTIC -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC")
>
> Not many a bug survives all those options. However my code for
> GetMostDerivedObject and GetTypeInfo compiles and runs just fine.

So ignoring whether it “works” or not, you should know that this is UB, and ubsan (-fsanitize=undefined) tries to
report all UB, even if the behavior may be sensible on actual hardware. That’s why I was surprised when you
said it ran cleanly.

Anyway, it looks like it’s a regression in clang: https://godbolt.org/z/dbW6KWKE8

Both typeid and dynamic_cast report an error in 20, but are silent in trunk :(

Filed https://github.com/llvm/llvm-project/issues/143065

—Oliver