cgen: fix if mut var != none { for optional interface values (fix #24351) (#24410)

vlib/v/gen/c/cgen.v

@@ -5269,8 +5269,17 @@ fn (mut g Gen) ident(node ast.Ident) {
 						is_option_unwrap := is_option && typ == node.obj.typ.clear_flag(.option)
 						cast_sym := g.table.sym(g.unwrap_generic(typ))
 						if obj_sym.kind == .interface && cast_sym.kind == .interface {
-							ptr := '*'.repeat(node.obj.typ.nr_muls())
+							if cast_sym.cname != obj_sym.cname {
-							g.write('I_${obj_sym.cname}_as_I_${cast_sym.cname}(${ptr}${node.name})')
+								ptr := '*'.repeat(node.obj.typ.nr_muls())
+								g.write('I_${obj_sym.cname}_as_I_${cast_sym.cname}(${ptr}${node.name})')
+							} else {
+								ptr := if is_option {
+									''
+								} else {
+									'*'.repeat(node.obj.typ.nr_muls())
+								}
+								g.write('${ptr}${node.name}')
+							}
 						} else {
 							mut is_ptr := false
 							if i == 0 {

vlib/v/tests/options/option_mut_if_none_test.v

@@ -0,0 +1,42 @@
+module main
+
+@[heap]
+interface IGameObject {
+mut:
+	name       string
+	parent     ?&IGameObject
+	next       ?&IGameObject
+	child      ?&IGameObject
+	last_child ?&IGameObject
+}
+
+@[heap]
+struct GameObject implements IGameObject {
+mut:
+	name       string
+	parent     ?&IGameObject
+	next       ?&IGameObject
+	child      ?&IGameObject
+	last_child ?&IGameObject
+}
+
+fn test_main() {
+	mut v1 := &GameObject{
+		name: 'v1'
+	}
+	v1.next = &GameObject{
+		name: 'v2'
+	}
+
+	mut next := v1.next
+	for {
+		if mut next != none {
+			eprintln(next.name)
+			assert next.name == 'v2'
+			next = next.next
+		} else {
+			break
+		}
+	}
+	assert next == none
+}