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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - dynamic_cast to ambiguous base produces the wrong answer"
href="https://bugs.llvm.org/show_bug.cgi?id=33439">33439</a>
</td>
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<th>Summary</th>
<td>dynamic_cast to ambiguous base produces the wrong answer
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>arthur.j.odwyer@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following class hierarchy:
#include <stdio.h>
struct Class2 { virtual ~Class2() {} };
struct Class3 { virtual ~Class3() {} };
struct Class4 : public Class3 {};
struct Class8 : public Class2, public virtual Class4 {};
struct Class9 : public Class4, public Class8 {};
int main() {
Class9 c9;
Class2 *c2 = static_cast<Class2 *>(&c9);
printf("cast Class2 to Class4: %p\n", dynamic_cast<Class4 *>(c2));
}
Clang correctly produces a warning diagnostic:
prog.cc:7:17: warning: direct base 'Class4' is inaccessible due to ambiguity:
struct Class9 -> struct Class4
struct Class9 -> struct Class8 -> struct Class4 [-Winaccessible-base]
struct Class9 : public Class4, public Class8 {};
^~~~~~~~~~~~~
1 warning generated.
But when you run the code, the dynamic_cast succeeds instead of failing as
expected.
cast Class2 to Class4: 0x7ffc2c41edd0
What we see there is the address of the inaccessible non-virtual base object,
NOT the address of the accessible virtual base object. (Not that the latter
would have been any more correct.) What we SHOULD see is a null pointer,
because dynamic_cast to an ambiguous base is supposed to fail.
GCC and MSVC get it right.
(This was caught by the same fuzzer as <a class="bz_bug_link
bz_status_NEW "
title="NEW - dynamic_cast from one virtual base to another produces the wrong answer"
href="show_bug.cgi?id=33425">bug 33425</a>, but it is a distinct bug
triggered under different conditions.)</pre>
</div>
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