C++ compiler should forbid throw move-only type

[fsb4000]

Hello.
We were reported a bug: microsoft/STL#2466
But we think that the report is wrong.

#include <exception>
#include <string>
using namespace std;

class C {
public:
  C() = default;
  C(const C&) = delete;
  C& operator=(const C&) = delete;
  C(C&&) = default;
  C& operator=(C&&) = default;

private:
  string m;
};

int main() {
  exception_ptr eptr;
  try {
    throw C();
  } catch (const C&) {
    eptr = std::current_exception();
  }
  try {
    rethrow_exception(eptr);
  } catch (const C&) {
  }
}

https://godbolt.org/z/q3cYGfneP

We think that the compiler should reject the program because the program violates [except.throw]/5: https://eel.is/c++draft/except.throw#5

When the thrown object is a class object, the constructor selected for the copy-initialization as well as the constructor selected for a copy-initialization considering the thrown object as an lvalue shall be non-deleted and accessible, even if the copy/move operation is elided

We decided to notify developers of other C++ compilers too.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[Quuxplusone]

This is https://quuxplusone.github.io/blog/2019/05/11/msvc-what-are-you-doing/ Basically, MSVC's ABI can't handle move-only exception types, but every other ABI in the world can. I would bet that it's not possible to forbid move-only exception types in Clang (because I bet some non-Windows customers rely on it), but it could certainly be made into a warning diagnostic, enabled with -Wall and/or -pedantic.

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Igor Zhukov (fsb4000)

Hello. We were reported a bug: https://github.com/microsoft/STL/issues/2466 But we think that the report is wrong.

#include <exception>
#include <string>
using namespace std;

class C {
public:
  C() = default;
  C(const C&) = delete;
  C& operator=(const C&) = delete;
  C(C&&) = default;
  C& operator=(C&&) = default;

private:
  string m;
};

int main() {
  exception_ptr eptr;
  try {
    throw C();
  } catch (const C&) {
    eptr = std::current_exception();
  }
  try {
    rethrow_exception(eptr);
  } catch (const C&) {
  }
}

https://godbolt.org/z/q3cYGfneP

We think that the compiler should reject the program because the program violates [except.throw]/5: https://eel.is/c++draft/except.throw#5

>When the thrown object is a class object, the constructor selected for the copy-initialization as well as the constructor selected for a copy-initialization considering the thrown object as an lvalue shall be non-deleted and accessible, even if the copy/move operation is elided

We decided to notify developers of other C++ compilers too.