typedef struct
{
} Product;

为一个 unnamed 结构声明一个名为 Product 的类型别名 - 但是你需要一个 named 结构用于你的前向声明 struct Product*next;，否则编译器无法确定它属于哪个定义.

Declares a type alias called Product for an unnamed struct - however you need a named struct for your forward declaration struct Product *next;, otherwise the compiler cannot determine which definition it belongs to.

最简单的解决方案是给结构一个名字:

The simplest solution is to give the struct a name:

typedef struct Product
{
    int value;
    struct Product *next;
} Product;