[jstimpfle]

> There's a well known limitation that mutable containers (and they're all mutable in Java) need to be invariant, but that doesn't make them unsound.

I see. Yeah I don't know precisely what these terms mean. If the creators of Generics mistakenly made them covariant, that only goes to show that maybe it's a little too complicated. IMHO.

To be more precise, what we want to do might be too complicated for practical (i.e. relatively simple) type systems to describe. So, why bother at all? Better learn how to structure programs simple enough to make them obviously correct (i.e. mistakes will be obvious and can be easily fixed). Instead of catering to the needs of impractical type systems. I think that's why C is still so popular: It removes most of the boilerplate (i.e. strides for array indexing, arithmetic operators, structs, other ABI things) but gets out of the users way if s/he needs to disregard these constraints for a while.

Even in C, there is a similar problem with const compatibility of pointers of more than 1 level of indirection. Example taken from [1]

    const int **pp2;
    int *p1;
    const int n = 13;
 
    pp2 = &p1; /* not allowed, but suppose it were */
    *pp2 = &n; /** valid, both const, but sets p1 to point at n */
    *p1 = 10;  /* valid, but changes const n */

[1] https://www.sanfoundry.com/c-tutorials-concept-pointer-compa...