fix

vlib/v/markused/walker.v

@@ -519,6 +519,7 @@ fn (mut w Walker) expr(node_ ast.Expr) {
 			if node.is_expr {
 				w.fn_by_name('free')
 			}
+			w.mark_by_type(w.table.find_or_register_thread(node.call_expr.return_type))
 			w.expr(node.call_expr)
 			w.uses_spawn = true
 

vlib/v/tests/concurrency/thread_type_test.v

@@ -0,0 +1,13 @@
+fn f() ! {
+	println('hi')
+}
+
+fn g() int {
+	println('hi')
+	return 23
+}
+
+fn test_main() {
+	_ = go f()
+	_ := spawn g()
+}