[llvm-bugs] [Bug 33487] New: dynamic_cast to ambiguous derived class produces the wrong answer

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33487

            Bug ID: 33487
           Summary: dynamic_cast to ambiguous derived class produces the
                    wrong answer
           Product: libc++abi
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

This is similar to bug 33439, but in that case we're casting between unrelated
types where the most derived type contains multiple (ambiguous) subobjects of
the destination type. In *this* case we're casting between related types — from
a base subobject to a derived type — where the source object is a subobject of
more than one object of the destination type. In both cases the symptom is
similar: dynamic_cast returns a pointer to the first such ambiguous subobject,
when it should be returning NULL.


#include <stdio.h>

struct Class1 { virtual ~Class1() {} };
struct Shared : public virtual Class1 {};
struct Class6 : public virtual Shared {};
struct Left : public Class6 {};
struct Right : public Class6 {};
struct Main : public Left, public Right {};

int main()
{
    Main m;
    Class1 *c1 = &m;
    Class6 *c6 = dynamic_cast<Class6*>(c1);
    printf("%p, should be nullptr\n", c6);
}

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