<html>
<head>
<base href="http://llvm.org/bugs/" />
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Lambda not allowed in a constant expression"
href="http://llvm.org/bugs/show_bug.cgi?id=20209">20209</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Lambda not allowed in a constant expression
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++11
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dyp-cpp@gmx.net
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Lambdas are allowed in constant expression if they are not evaluated. This
allows the trick PYTHY uses (allegedly by Richard Smith, see
<a href="http://pfultz2.github.io/Pythy/">http://pfultz2.github.io/Pythy/</a> ):
template<class T>
T* address(T&& t)
{ return &t; }
//template<class T>
struct c
{
static constexpr auto* x = false ? address([]{}) : nullptr;
};
int main()
{
(*c::x)();
//(*c<int>::x)();
}
While this codes compiles with trunk 212348, replacing the class with a class
template results in the following error message:
error: a lambda expression may not appear inside of a constant expression
static constexpr auto* x = false ? address([]{}) : nullptr;
^
note: in instantiation of template class 'c<int>' requested here
(*c<int>::x)();
^
1 error generated.
It is not clear to me why using a class template should make a difference in
this case.
This bug might be related to #16501 and CWG DR 1607
<a href="http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1607">http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#1607</a></pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>