checker: allow none to be casted to all option types, including aliases (#17348)

vlib/v/checker/checker.v

@@ -2755,7 +2755,9 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	if from_type == ast.void_type {
 		c.error('expression does not return a value so it cannot be cast', node.expr.pos())
 	}
-	if to_sym.kind == .sum_type {
+	if to_type.has_flag(.option) && from_type == ast.none_type {
+		// allow conversion from none to every option type
+	} else if to_sym.kind == .sum_type {
 		if from_type in [ast.int_literal_type, ast.float_literal_type] {
 			xx := if from_type == ast.int_literal_type { ast.int_type } else { ast.f64_type }
 			node.expr_type = c.promote_num(node.expr_type, xx)

vlib/v/tests/cast_none_to_option_test.v

@@ -0,0 +1,38 @@
+struct Test {}
+
+type AliasType = Test
+
+type I20 = int
+
+type Byt = u8
+
+fn test_cast_none() {
+	a := ?I20(none)
+	assert a == none
+	b := ?int(none)
+	assert b == none
+	c := ?i16(none)
+	assert c == none
+	d := ?u32(none)
+	assert d == none
+	e := ?u16(none)
+	assert e == none
+	f := ?u8(none)
+	assert f == none
+	g := ?Test(none)
+	assert g == none
+	h := ?AliasType(none)
+	assert h == none
+	i := ?Byt(none)
+	assert i == none
+	j := ?rune(none)
+	assert j == none
+	k := ?string(none)
+	assert k == none
+	l := ?[]Byt(none)
+	assert l == none
+	m := ?[]Test(none)
+	assert m == none
+	n := ?[]AliasType(none)
+	assert n == none
+}