cgen: fix if sumtype var is none (#15496)

vlib/v/gen/c/infix.v

@@ -619,7 +619,11 @@ fn (mut g Gen) infix_expr_is_op(node ast.InfixExpr) {
 	} else if left_sym.kind == .sum_type {
 		g.write('_typ $cmp_op ')
 	}
-	g.expr(node.right)
+	if node.right is ast.None {
+		g.write('$ast.none_type.idx() /* none */')
+	} else {
+		g.expr(node.right)
+	}
 }
 
 fn (mut g Gen) gen_interface_is_op(node ast.InfixExpr) {

vlib/v/tests/inout/printing_sumtype_with_none.out

@@ -0,0 +1,3 @@
+first field is a string
+second field is none
+third field is None

vlib/v/tests/inout/printing_sumtype_with_none.vv

@@ -0,0 +1,35 @@
+module main
+
+struct None {}
+
+struct MyStruct {
+	my_first_field  string|none
+	my_second_field string|none
+	my_third_field  string|None
+}
+
+fn main() {
+	s := MyStruct{
+		my_first_field: 'blah'
+		my_second_field: none
+		my_third_field: None{}
+	}
+
+	println(if s.my_first_field is string {
+		'first field is a string'
+	} else {
+		'first field is none'
+	})
+
+	println(if s.my_second_field is none {
+		'second field is none'
+	} else {
+		'second field is a string'
+	})
+
+	println(if s.my_third_field is None {
+		'third field is None'
+	} else {
+		'third field is a string'
+	})
+}