[libcxx/test] Fix UB when deleting D through a pointer to B.

[• Quuxplusone]

Summary

After I apply Lénárd Szolnoki's proposal to disable implicit conversion of default_delete<D> to default_delete<B>, this particular std::optional test begins to fail:

/llvm-project/libcxx/test/std/utilities/optional/optional.object/optional.object.assign/optional_U.pass.cpp:243:13: error: no viable overloaded '='
        opt = std::move(other);
        ~~~ ^ ~~~~~~~~~~~~~~~~
/llvm-project/libcxx/include/optional:768:41: note: candidate function not viable: no known conversion from 'optional<unique_ptr<D, default_delete<D>>>' to 'optional<unique_ptr<B, default_delete<B>>>' for 1st argument
    _LIBCPP_INLINE_VISIBILITY optional& operator=(optional&&) = default;
                                        ^

I believe this is a true positive. The offending code in this test is circa line 243:

https://github.com/llvm/llvm-project/blob/master/libcxx/test/std/utilities/optional/optional.object/optional.object.assign/optional_U.pass.cpp#L240-L248

{
    optional<std::unique_ptr<B>> opt;
    optional<std::unique_ptr<D>> other(new D());
    opt = std::move(other);
    assert(static_cast<bool>(opt) == true);
    assert(static_cast<bool>(other) == true);
    assert(opt->get() != nullptr);
    assert(other->get() == nullptr);
}

At the closing curly brace, we destroy opt, which deletes opt->get() (a B* which dynamically points to a D object), which has UB because http://eel.is/c++draft/expr.delete#3 — am I right?

Giving B a virtual dtor fixes the UB without affecting anything else about this test.

[ldionne]

Your reasoning looks right to me. Thanks for fixing.

[• Quuxplusone]

Awesome. (As usual @ldionne you'll have to land this for me.)

libcxx/test/std/utilities/optional/optional.object/optional.object.assign/optional_U.pass.cpp

 
 struct Y2
 {
     Y2() = default;
     Y2(const int&) = delete;
     Y2& operator=(const int&) { return *this; }
 };
 
-class B {};
+struct B { virtual ~B() = default; };
 class D : public B {};
 
 
 template <class T>
 struct AssignableFrom {
   static int type_constructed;
   static int type_assigned;
 static int int_constructed;