Crystal does some of what you want:

https://tio.run/##ZVDBbsIwDL33KywmrRcaUXbbhHbeaYdpJ4RQaA1kCk...

Note how the language is pretty smart about flow-typing variables as you narrow down their types. `x` here is typed as a tagged union (String | Int32 | Array(Char) | Float64 | UInt32), but by the time the program gets to the final `else` the compiler has figured out that the String and Array cases have already been dealt with (even though we never mentioned Array by name, just asked if the object implements a method that only Array implements), so it lets us call `x.abs`, which only works for numeric types. Of course, `abs` has different implementations for Int32 and Float64, but the signatures are compatible so the compiler lets it fly, resorting to dynamic dispatch at runtime.

The missing part is that there's nothing like a CAN_SAFELY_BECOME_A operator so I had to implement that test myself (as `try_to_u32`) and call it early to get the flow typing to work.

The Wikipedia article for the concept is https://en.wikipedia.org/wiki/Flow-sensitive_typing