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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - declval() can be implemented without requiring a template instantiation."
href="https://llvm.org/bugs/show_bug.cgi?id=27798">27798</a>
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<th>Summary</th>
<td>declval() can be implemented without requiring a template instantiation.
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<th>Product</th>
<td>libc++
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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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
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<th>Reporter</th>
<td>eniebler@boost.org
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>declval gets used *everywhere*. It doesn't need to instantiate a template.
Rather than:
template <class _Tp>
typename add_rvalue_reference<_Tp>::type
declval() noexcept;
declval can be defined as:
template<typename _Tp, typename _Up = _Tp&&>
_Up
__declval(int);
template<typename _Tp>
_Tp
__declval(long);
template<typename _Tp>
auto declval() noexcept -> decltype(__declval<_Tp>(0));
In a large code base that makes heavy use of templates, this small change is a
measured 2% compile-time win.
I think the as-if rule gives implementers enough latitude to make this change.</pre>
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