cgen: fix thread return type generation (fix #20836) (#20850)

vlib/v/gen/c/spawn_and_go.v

@@ -122,9 +122,8 @@ fn (mut g Gen) spawn_and_go_expr(node ast.SpawnExpr, mode SpawnGoMode) {
 			gohandle_name = '__v_thread'
 		}
 	} else {
-		opt := if is_opt { '${option_name}_' } else { '' }
-		res := if is_res { '${result_name}_' } else { '' }
-		gohandle_name = '__v_thread_${opt}${res}${g.table.sym(g.unwrap_generic(node.call_expr.return_type)).cname}'
+		ret_styp := g.typ(g.unwrap_generic(node.call_expr.return_type)).replace('*', '_ptr')
+		gohandle_name = '__v_thread_${ret_styp}'
 	}
 	if is_spawn {
 		if g.pref.os == .windows {

vlib/v/tests/thread_ptr_ret_test.v

@@ -0,0 +1,21 @@
+fn test_main() {
+	foo := spawn get_pointer()
+	ret := foo.wait()
+	assert *ret == 42
+}
+
+fn test_opt() {
+	foo := spawn get_pointer_opt()
+	ret := foo.wait()
+	assert *ret? == 42
+}
+
+fn get_pointer() &int {
+	val := 42
+	return &val
+}
+
+fn get_pointer_opt() ?&int {
+	val := 42
+	return &val
+}