cgen: fix return of mut symtype (#13214)

vlib/v/gen/c/cgen.v

@@ -5004,6 +5004,8 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 			if g.fn_decl.return_type.is_ptr() {
 				var_str := g.expr_string(expr0)
 				g.write(var_str.trim('&'))
+			} else if g.table.sym(g.fn_decl.return_type).kind in [.sum_type, .interface_] {
+				g.expr_with_cast(expr0, node.types[0], g.fn_decl.return_type)
 			} else {
 				g.write('*')
 				g.expr(expr0)

vlib/v/tests/fn_return_mut_sumtype_test.v

@@ -0,0 +1,23 @@
+type Sum = Struct | int
+
+struct Struct {
+mut:
+	value int
+}
+
+fn update(mut s Struct) Sum {
+	s.value += 1
+	return s
+}
+
+fn test_fn_return_mut_sumtype() {
+	mut s := Sum(Struct{
+		value: 1
+	})
+	if mut s is Struct {
+		s = update(mut s)
+		assert s.value == 2
+	} else {
+		assert false
+	}
+}