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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - std::is_base_of should give correct result for incomplete unions"
href="https://bugs.llvm.org/show_bug.cgi?id=41843">41843</a>
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<th>Summary</th>
<td>std::is_base_of should give correct result for incomplete unions
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>8.0
</td>
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<th>Hardware</th>
<td>Macintosh
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>alisdairm@me.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=21928" name="attach_21928" title="Source for failing test">attachment 21928</a> <a href="attachment.cgi?id=21928&action=edit" title="Source for failing test">[details]</a></span>
Source for failing test
C++20 clarifies the expectation for detecting traits that cannot determine a
result due to an incomplete class. However, is_base_of is expected to
recognize that unions can never have a base class, nor ever be a base class,
and so give the correct result - per the current working draft.
The following code is expected to compile without triggering a static assert.
For the current clang/libc++, it fails to compile the middle two heterogeneous
assertions:
#include <type_traits>
struct Incomplete;
union Uncomplete;
int main() {
static_assert( std::is_base_of_v<Incomplete, Incomplete>);
static_assert(!std::is_base_of_v<Incomplete, Uncomplete>); // fails to
compile
static_assert(!std::is_base_of_v<Uncomplete, Incomplete>); // fails to
compile
static_assert(!std::is_base_of_v<Uncomplete, Uncomplete>);
}</pre>
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