<html>
<head>
<base href="https://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 --- - Cannot assign a lambda expression to an optional<function<...>>"
href="https://llvm.org/bugs/show_bug.cgi?id=31071">31071</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Cannot assign a lambda expression to an optional<function<...>>
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</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++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>matt.rajca@me.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=17615" name="attach_17615" title="Sample project">attachment 17615</a> <a href="attachment.cgi?id=17615&action=edit" title="Sample project">[details]</a></span>
Sample project
Suppose we a function that takes a completion handler in the form of an
optional<function>:
using Handler = function<void(void)>;
void doSomething(int flags, optional<Handler> callback =
optional<Handler>()) {
if (callback) {
(*callback)();
}
}
Since a default value of an empty optional is provided, I can invoke this
without passing a value for the final parameter just fine:
doSomething(0);
Moreover, I can invoke `doSomething` if I explicitly cast a lambda expression
to a `Handler` and pass it for the final parameter:
doSomething(0, Handler([]{
cout << "hello world" << endl;
}));
However, passing a lambda expression directly does not work (the following does
not compile with "No matching function for call to 'doSomething'"):
doSomething(0, []{
cout << "hello world" << endl;
});
I assume this is because the type checker doesn't know we can assign a lambda
expression to an optional (even if it's an optional<function> and lambda
expression can be assigned to functions).
In other words, if a lambda expression L can be assigned to a function F, we
should be able to assign L to an optional of type optional<F>.
A sample project that demonstrates this is attached. I am testing with Xcode
8.1 GM.</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>