This is a compiler bug.

When template parameter C is deduced from the call handler(safeCallback), the default argument `= "hunter2" should be stripped from the deduced function pointer type.

Then, the call callback("John"); in handler template function body should fail to compile always, because void function(string, string) is not callable using one string argument.

Kenji Hara

2014-04-29 19:38 GMT+09:00 Andrej Mitrovic via Digitalmars-d <digitalmars-d@puremagic.com>:

-----
import std.traits;
import std.stdio;

void handler(C)(C callback)
{
callback("John");
}

void main()
{
auto safeCallback = (string user, string pass = "hunter2")
{
writefln("The password is: '%s'", pass);
};

handler(safeCallback);
someOtherFunc();
}

void someOtherFunc()
{
auto hijackPassword = (string user, string pass)
{
writefln("Now I know your password: '%s'", pass);
};

handler(hijackPassword);
}
-----