> as long as the struct tags are the same

Exactly. Now you have a naming problem. You need a naming convention that every user of the list library must follow, or else their types will be incompatible. And what about typedefs? If A is a typedef of B, or more generally A and B are typedef-related (their normal forms, obtained by following all typedefs, are the same), lists of A and B will be incompatible unless users agree on a common name. The only realistic choice is the normal form, but then this actively works against the abstraction provided by typedef.

And this is just for types. What about functions? While it is legal to do identical definitions of struct list_int, it is not for list_int_init() and list_int_add(). Or global variables: it is legal to do several identical extern declarations, but there can only be one definition; which compilation unit gets to do it?

> Now you have a naming problem. You need a naming convention that every user of the list library must follow, or else their types will be incompatible.

Oh, that's simple: just have empty struct tags.

> And what about typedefs?

The names introduced by the typedefs are irrelevant.

> A and B are typedef-related (their normal forms, obtained by following all typedefs, are the same), lists of A and B will be incompatible unless users agree on a common name.

Huh?

    typedef struct { int x; } A;
    typedef struct { int x; } B;

    typedef struct { header_list header; A payload; } list_of_A;
    typedef struct { header_list header; B payload; } list_of_B;