cgen: fix unwrapped option selector assigning (#20816)

vlib/v/gen/c/cgen.v

@@ -3670,7 +3670,7 @@ fn (mut g Gen) selector_expr(node ast.SelectorExpr) {
 	if is_as_cast {
 		g.write('(')
 	}
-	if node.or_block.kind != .absent && !g.is_assign_lhs && g.table.sym(node.typ).kind != .chan {
+	if node.or_block.kind != .absent && g.table.sym(node.typ).kind != .chan {
 		is_ptr := sym.kind in [.interface_, .sum_type]
 		stmt_str := g.go_before_last_stmt().trim_space()
 		styp := g.typ(g.unwrap_generic(node.typ))

vlib/v/tests/option_selector_assign_test.v

@@ -0,0 +1,20 @@
+struct Tree {
+mut:
+	nr_elems int
+	parent   ?&Tree
+}
+
+fn (mut t Tree) set_nr_elems(name string, value int) {
+	t.parent or { return }.nr_elems = value
+}
+
+fn test_main() {
+	parent := Tree{
+		nr_elems: 11
+	}
+	mut child := Tree{
+		parent: &parent
+	}
+	child.set_nr_elems('Buzz', 123)
+	assert child.parent or { return }.nr_elems == 123
+}