isHandlerAddable

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

static assert(isHandlerAddable!(void function()[], void function()));
static assert(isHandlerAddable!(void delegate()[], void delegate()));
static assert(isHandlerAddable!(CallableStruct[],  CallableStruct));
static assert(isHandlerAddable!(CallableClass[],   CallableClass));
static assert(isHandlerAddable!(void delegate()[], void function()));
static assert(isHandlerAddable!(void delegate()[], CallableStruct));
static assert(isHandlerAddable!(void delegate()[], CallableClass));