{.union_type奇怪的语法

Question

我正在浏览一篇有关c中的联合的文章，而我遇到了这个问题：

Union of structs with common first member

我不是C语言的初学者，但看不到语法“ {。”在哪里解释或在任何地方都是正确的？

union container_storage a =
    {.container_a = {.contained = {.type = type_a}, .test = 42}};