I think I see what you’re getting at. If I create an empty mutable list and send a reference to it to two different threads A and B, where A adds an integer to the list and B adds a string, then it’s nondeterministic which thread raises a type error.
And degrading the type to a union (int ∨ string) as I mentioned is still nondeterministic even though unions form a lattice—I can recover determinism when writing to the list in both A and B, but supposing A writes first, B writes second, and then A reads expecting only integers, it gets a type error because it hasn’t checked for the string case.
But I’d argue that 1. this is bad form and you want a type error somewhere and 2. you have this problem already with type-changing mutable references in a dynamically typed multithreaded language. (Say just “int” and “string” instead of “list of int” and “list of string”.)
I need a container type that can be empty, because I need the monomorphic type of the cell to be undecided until the reference has been handed out to the worker threads.
If your program doesn't have a race condition, then you're fine. And if it does, then the list being monomorphic is not going to help (try your scenario with a C++ vector or Java ArrayList).
If your standard for a possible programming language that supports concurrency is that no data races are possible, then there's very few languages to be had.
This particular discussion wasn't about ruling out data races (although, of course, that is important too). It was about ruling out trying to read a list of strings from a reference cell containing a list of ints.
You're the one who brought up threads as a reason runtime monomorphisation doesn't work. If access to the list is properly synchronized, then what I proposed works just fine (you get your runtime error in whatever thread accesses the list second). If it's not, then unless you're using a concurrent data structure you're already screwed. The type system will at most change exactly how you get screwed.
Say, because you want to use that reference cell in two different threads.