Clang typeinfo / dynamic_cast issue

README.md

Clang dynamic_cast fails across shared library boundary

A derived polymorphic template class that is final which is created in a shared library cannot be dynamic_cast to its base class in another TU.

$ cmake -S . -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo
$ cmake --build build && build/DowncastRepro
[4/4] Linking CXX executable DowncastRepro
dynamic_cast non-final: 0x600001164030
dynamic_cast final:     0x0
dynamic_cast final loc: 0x600003aa8000

class.cpp

#include "class.hpp"

std::unique_ptr<Base> create() { return std::make_unique<Derived<int>>(); }

std::unique_ptr<Base> createFinal() {
  return std::make_unique<DerivedFinal<int>>();
}

const std::type_info &info() { return typeid(Derived<int>); }
const std::type_info &infoFinal() { return typeid(DerivedFinal<int>); }

class.hpp

#pragma once

#include <memory>
#include <typeinfo>

struct Base {
  virtual ~Base() = default;
};

template <typename T> struct DerivedFinal final : public Base {};
template <typename T> struct Derived : public Base {};

std::unique_ptr<Base> create();
std::unique_ptr<Base> createFinal();

CMakeLists.txt

cmake_minimum_required(VERSION 3.14)
project(DowncastRepro LANGUAGES CXX)

add_library(Foo SHARED class.cpp)
target_include_directories(Foo PUBLIC .)
target_compile_features(Foo PUBLIC cxx_std_20)

add_executable(DowncastRepro main.cpp)
target_link_libraries(DowncastRepro PRIVATE Foo)

main.cpp

#include <iostream>

#include "class.hpp"

int main() {
  auto base = create();
  auto *derived = dynamic_cast<Derived<int> *>(base.get());
  std::cout << "dynamic_cast non-final: " << derived << std::endl;

  base = createFinal();
  auto *derivedFinal = dynamic_cast<DerivedFinal<int> *>(base.get());
  std::cout << "dynamic_cast final:     " << derivedFinal << std::endl;

  base = std::make_unique<DerivedFinal<int>>();
  auto *derivedFinalLocal = dynamic_cast<DerivedFinal<int> *>(base.get());
  std::cout << "dynamic_cast final loc: " << derivedFinalLocal << std::endl;
}