isHandler

static struct CallableStruct { void opCall(){} }
static class CallableClass { void opCall(){} }

static assert(isHandler!(void function()));
static assert(isHandler!(void delegate()));
static assert(isHandler!(CallableClass));
static assert(isHandler!(CallableStruct));
static assert(isHandler!(void function()[]));
static assert(isHandler!(void delegate()[]));
static assert(isHandler!(CallableStruct[]));
static assert(isHandler!(CallableClass[]));
void delegate(Exception)[] handler = void;
static assert(isHandler!(void delegate(Exception)[]));