ubsan has stopped identifying a mismatch between static and dynamic type of an argument to typeid() and dynamic_cast

[ojhunt]

A post on C++'s std-proposals said some clearly UB code did not trigger a sanitizer failure, which was somewhat confusion, but after investigating it's a regression since clang 20.

#include <typeinfo>
struct Foo {
   virtual ~Foo(){}
};
struct Bar {
   virtual ~Bar(){}
};

struct Subclass : Bar {};

int main(int arc, char**) {
   Foo f;
   Bar *b = reinterpret_cast<Bar*>(&f);
   (void)typeid(*b);
   (void)dynamic_cast<Subclass*>(b);
   return 0;
}

Compiling with -fsanitize=undefined and running reports errors for the dynamic_cast and the typeid in 20, but is silent in trunk

[chrchr-github]

https://godbolt.org/z/Tjr4j6E48