And if you want to go weirder, implicit int means that you can leave out the `int`:

    typedef x;

Even "better", C declarators can be empty in order to allow for struct/enum declarations that do not list any variables. So you can leave out the variable, add qualifiers, etc:

    typedef;
    const typedef;
    typedef const;

It's a really fun syntax in some ways.

For the curious on why this is: at the syntax level, typedef is treated like any other storage class specifier (static, extern, auto, register). So it is legal just like 'int static a' is legal.