cgen: fix or {} handling, when waiting for a single go thread, of a function returning !Type (fix #16065) (#16073)

vlib/v/gen/c/fn.v

@@ -1885,12 +1885,20 @@ fn (mut g Gen) go_expr(node ast.GoExpr) {
 		g.writeln('$arg_tmp_var->ret_ptr = malloc(sizeof($s_ret_typ));')
 	}
 	is_opt := node.call_expr.return_type.has_flag(.optional)
+	is_res := node.call_expr.return_type.has_flag(.result)
 	mut gohandle_name := ''
 	if node.call_expr.return_type == ast.void_type {
-		gohandle_name = if is_opt { '__v_thread_Option_void' } else { '__v_thread' }
+		if is_opt {
+			gohandle_name = '__v_thread_Option_void'
+		} else if is_res {
+			gohandle_name = '__v_thread_Result_void'
+		} else {
+			gohandle_name = '__v_thread'
+		}
 	} else {
 		opt := if is_opt { '${option_name}_' } else { '' }
-		gohandle_name = '__v_thread_$opt${g.table.sym(g.unwrap_generic(node.call_expr.return_type)).cname}'
+		res := if is_res { '${result_name}_' } else { '' }
+		gohandle_name = '__v_thread_$opt$res${g.table.sym(g.unwrap_generic(node.call_expr.return_type)).cname}'
 	}
 	if g.pref.os == .windows {
 		simple_handle := if node.is_expr && node.call_expr.return_type != ast.void_type {

vlib/v/tests/go_wait_option_test.v

@@ -80,3 +80,29 @@ fn test_array_val_interate() {
 	assert res[1] == 0.0
 	assert res[2] == 1.5
 }
+
+// For issue 16065
+fn get_only_a_option_return(return_none bool) ? {
+	if return_none {
+		return
+	}
+	return error('msg')
+}
+
+fn get_only_a_result_return() ! {
+	return error('msg')
+}
+
+fn test_only_a_option_return() {
+	t1 := go get_only_a_option_return(true)
+	t1.wait() or { assert false }
+	t2 := go get_only_a_option_return(false)
+	t2.wait() or { assert true }
+	assert true
+}
+
+fn test_only_a_result_return() {
+	t := go get_only_a_result_return()
+	t.wait() or { assert true }
+	assert true
+}