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  <!--*********************************************************************-->
  <h1>Type traits intrinsic design</h1>
  <!--*********************************************************************-->

<p>
This is a survey of the type traits intrinsics clang has, and those needed.
The names and definitions of several of the needed type traits has recently
changed.  Please see:
<a href="https://wg21.link/n3142">N3142</a>.
</p>

<blockquote>
<table border="1">
<caption>Legend</caption>

<tr>
<td>clang supplies it and it is absolutely necessary</td>
<td bgcolor="#80FF80"><tt>some_trait(T)</tt></td>
</tr>

<tr>
<td>clang supplies it and it is useful</td>
<td bgcolor="#96B9FF"><tt>some_trait(T)</tt></td>
</tr>

<tr>
<td>clang supplies it and it is not needed</td>
<td><tt>some_trait(T)</tt></td>
</tr>

<tr>
<td>clang does not supply it and it is not needed</td>
<td></td>
</tr>

<tr>
<td>clang does not supply it and it is absolutely necessary</td>
<td bgcolor="#FF5965"><tt>some_trait(T)</tt></td>
</tr>

</table>

<p></p>

<table border="1">
<caption>Needed type traits vs clang type traits</caption>

<tr>
<th>libc++ Needs</th>
<th>clang Has</th>
</tr>

<tr>
<td><tt>is_union&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_union(T)</tt></td>
</tr>

<tr>
<td><tt>is_class&lt;T&gt;</tt></td>
<td bgcolor="#96B9FF"><tt>__is_class(T)</tt></td>
</tr>

<tr>
<td><tt>is_enum&lt;T&gt;</tt></td>
<td bgcolor="#96B9FF"><tt>__is_enum(T)</tt></td>
</tr>

<tr>
<td><tt>is_pod&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_pod(T)</tt></td>
</tr>

<tr>
<td><tt>has_virtual_destructor&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__has_virtual_destructor(T)</tt></td>
</tr>

<tr>
<td><tt>is_constructible&lt;T, Args...&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_default_constructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_copy_constructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_move_constructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_assignable&lt;T, U&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_copy_assignable&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_move_assignable&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_destructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_trivially_constructible&lt;T, Args...&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_trivially_constructible(T, U)</tt></td>
</tr>

<tr>
<td><tt>is_trivially_default_constructible&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__has_trivial_constructor(T)</tt></td>
</tr>

<tr>
<td><tt>is_trivially_copy_constructible&lt;T&gt;</tt></td>
<td><tt>__has_trivial_copy(T)</tt></td>
</tr>

<tr>
<td><tt>is_trivially_move_constructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_trivially_assignable&lt;T, U&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_trivially_assignable(T, U)</tt></td>
</tr>

<tr>
<td><tt>is_trivially_copy_assignable&lt;T&gt;</tt></td>
<td><tt>__has_trivial_assign(T)</tt></td>
</tr>

<tr>
<td><tt>is_trivially_move_assignable&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_trivially_destructible&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__has_trivial_destructor(T)</tt></td>
</tr>

<tr>
<td><tt>is_nothrow_constructible&lt;T, Args...&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_nothrow_default_constructible&lt;T&gt;</tt></td>
<td><tt>__has_nothrow_constructor(T)</tt></td>
</tr>

<tr>
<td><tt>is_nothrow_copy_constructible&lt;T&gt;</tt></td>
<td><tt>__has_nothrow_copy(T)</tt></td>
</tr>

<tr>
<td><tt>is_nothrow_move_constructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_nothrow_assignable&lt;T, U&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_nothrow_copy_assignable&lt;T&gt;</tt></td>
<td><tt>__has_nothrow_assign(T)</tt></td>
</tr>

<tr>
<td><tt>is_nothrow_move_assignable&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_nothrow_destructible&lt;T&gt;</tt></td>
<td></td>
</tr>

<tr>
<td><tt>is_trivial&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_trivial(T)</tt></td>
</tr>

<tr>
<td><tt>is_trivially_copyable&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_trivially_copyable(T)</tt></td>
</tr>

<tr>
<td><tt>is_standard_layout&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_standard_layout(T)</tt></td>
</tr>

<tr>
<td><tt>is_literal_type&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_literal_type(T)</tt></td>
</tr>

<tr>
<td><tt>is_convertible&lt;T, U&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_convertible_to(T, U)</tt></td>
</tr>

<tr>
<td><tt>is_base_of&lt;T, U&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__is_base_of(T, U)</tt></td>
</tr>

<tr>
<td><tt>underlying_type&lt;T&gt;</tt></td>
<td bgcolor="#80FF80"><tt>__underlying_type(T)</tt></td>
</tr>

<tr>
<td><tt>is_polymorphic&lt;T&gt;</tt></td>
<td><tt>__is_polymorphic(T)</tt></td>
</tr>

<tr>
<td><tt>is_empty&lt;T&gt;</tt></td>
<td><tt>__is_empty(T)</tt></td>
</tr>

<tr>
<td><tt>is_abstract&lt;T&gt;</tt></td>
<td><tt>__is_abstract(T)</tt></td>
</tr>

</table>
</blockquote>

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