expect

assert(ok(42).expect("oops") == 42);
ok().expect("oops"); // void value

version (D_Exceptions)
{
    () @trusted
    {
        assert(collectExceptionMsg!Throwable(Expected!int.init.expect("oops")) == "oops: empty");
        assert(collectExceptionMsg!Throwable(err!int("foo").expect("oops")) == "oops: foo");
    }();
}

assert(ok("foo").expect!(a => "bar") == "foo");
assert(err!string("foo").expect!(a => "bar") == "bar");
assert(err!string("foo").expect!((a) {}) is null);

static struct NonCopyable { @disable this(this); int foo; }
assert(err!NonCopyable(42).expect!((e) {}) == NonCopyable.init);
err!void(42).expect!((e) {});
ok!int().expect!(e => assert(0));
ok!int(42).expect!(e => assert(0));