No, you are right, but it is not "in separate files" but "translation units". Two distinct list(int) from two different libraries need to be compatible when used together, which means you would include the header of those two libraries and then both end up in the same translation unit.
C23 relaxed requirements for types with tag, this works for structures with tag, but for type generic structures you then need to synthesize a tag that depends on the type, e.g. list_int, list_float, etc.. so that differently parametrized types do not collide. This works quite well in practice, but is not perfect.
I don't think that's the scenario the other commenter was talking about:
there is no centralized monomorphization. If an application uses two libraries, each of which handles lists of int, each library will have to independently macro-generate its separate list implementation, and because C's type system is nominal, the generated types will be isomorphic but incompatible.
So, lib_a.c includes header_only_list.h, and lib_b.c also includes the same header_only_list.h, but they can't they pass the list structures between each other because those structs would be incompatible even though they're textually identical ("the generated types will be isomorphic but incompatible"). To which I replied that no, they would not, no C program would be able to work if this were true.
Otherwise, the mentioning of monomorphization doesn't make any sense: of course two completely different implementations of lists will be incompatible.
But "the generated types will be isomorphic but incompatible" is right. It is ok if you have lib_a.c and lib_b.c because these are two separate translation units. But this only works if both of these libraries use the list type internally and pass the pointer to each other as a void pointer. The moment you have two headers lib_a.h lib_b.h both including header_only_list.h and defining an API using a generic type list(int) and where you then try to also include lib_b.h from lib_a.c it does not work.
Comments
No, you are right, but it is not "in separate files" but "translation units". Two distinct list(int) from two different libraries need to be compatible when used together, which means you would include the header of those two libraries and then both end up in the same translation unit.
C23 relaxed requirements for types with tag, this works for structures with tag, but for type generic structures you then need to synthesize a tag that depends on the type, e.g. list_int, list_float, etc.. so that differently parametrized types do not collide. This works quite well in practice, but is not perfect.
I don't think that's the scenario the other commenter was talking about:
So, lib_a.c includes header_only_list.h, and lib_b.c also includes the same header_only_list.h, but they can't they pass the list structures between each other because those structs would be incompatible even though they're textually identical ("the generated types will be isomorphic but incompatible"). To which I replied that no, they would not, no C program would be able to work if this were true.
Otherwise, the mentioning of monomorphization doesn't make any sense: of course two completely different implementations of lists will be incompatible.
But "the generated types will be isomorphic but incompatible" is right. It is ok if you have lib_a.c and lib_b.c because these are two separate translation units. But this only works if both of these libraries use the list type internally and pass the pointer to each other as a void pointer. The moment you have two headers lib_a.h lib_b.h both including header_only_list.h and defining an API using a generic type list(int) and where you then try to also include lib_b.h from lib_a.c it does not work.