connectionpool can't compile any more

[soarowl]

I have snippet:

import vibe.core.connectionpool, vibe.core.net;

void main()
{
    auto m_pool = new ConnectionPool!TCPConnection({
        auto conn = connectTCP("localhost", 8080);
        conn.keepAlive = true;
        return conn;
    }, 10);

    auto conn = m_pool.lockConnection;
    conn.write("hello");
}

It compile under 0.8.2 but not 0.8.3. Has following error messages:

C:\Users\znw\AppData\Roaming\dub\packages\vibe-core-1.4.0\vibe-core\source\vibe\core\connectionpool.d(93,45): Error: template instance vibe.core.net.TCPConnection.opCast!(void*) does not match template declaration opCast(T)() if (is(T == bool))
C:\Users\znw\AppData\Roaming\dub\packages\vibe-core-1.4.0\vibe-core\source\vibe\core\connectionpool.d(179,11): Error: generated function vibe.core.net.TCPConnection.opAssign (TCPConnection p) is not callable using argument types (typeof(null))
C:\Users\znw\AppData\Roaming\dub\packages\vibe-core-1.4.0\vibe-core\source\vibe\core\connectionpool.d(148,10): Error: incompatible types for ((conn) !is (null)): 'TCPConnection' and 'typeof(null)'
source\app.d(5,23): Error: template instance vibe.core.connectionpool.ConnectionPool!(TCPConnection) error instantiating
C:\Users\znw\AppData\Roaming\dub\packages\vibe-core-1.4.0\vibe-core\source\vibe\core\connectionpool.d(161,53): Error: template instance vibe.core.net.TCPConnection.opCast!(void*) does not match template declaration opCast(T)() if (is(T == bool))
C:\Users\znw\AppData\Roaming\dub\packages\vibe-core-1.4.0\vibe-core\source\vibe\core\connectionpool.d(72,2): Error: template instance vibe.core.connectionpool.LockedConnection!(TCPConnection) error instantiating
source\app.d(5,23):        instantiated from here: ConnectionPool!(TCPConnection)
D:\ldc64\bin\ldc2.exe failed with exit code 1.

When I change to dmd 2.79.0, it can't compiled either.

test.zip