Interface function implementations accept T * as a first parameter, whereas a corresponding function in a virtual table accepts void *, thereby making these two function types incompatible (godbolt).

Moreover (6.2.7 Compatible type and composite type):

All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.

And I am curious about this code:

TypeclassName T_to_TypeclassName(T* x)
{
    static TypeclassName_vtable const tc = {.func_name = (ReturnType (*const)(void*, ...))(T_func_name) };
    return (TypeclassName){.tc = &tc, .self = x};
}

Here, .func_name and T_func_name are two declarations to the same function having incompatible types. Then we cast T_func_name to the type accepting void * as a first parameter; we cast incompatible types. Does it even follow the standard, or it is UB?

The C99 draft standard: http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf