Go, like Python, doesn't have such a distinction, thingvalue could be an array item, a map (dict, or hash) access, or access to a char in a string. In those languages, it's the operand that defines which operation to use

Again, no.

Go is a strongly typed language, every variable has a well-defined type associated it. You can have a nested map type c := map[string]map[string]int, in which case you (and the compiler) can know that its element c["key"] is of type map[string]int and the sub-elements of that (c["key"]["subkey"]) are of type int. Assigning a value of any other type to those keys would result in a compile time error.

So in theory, you could have autovivification in the language - but in practice you can't, you have to explicitly initialize the mid-level maps yourself, as explained by this blog post: Go maps in action.

The underlying reason for this, I think, is that Go differentiates between a nil map and an empty map. The zero value of a(n uninitialized) map is a nil map, you have to initialize yourself with make. A nil map can be safely read from, but assigning any key to it results in a runtime panic. When you declare a nested map as the example above, the mid-level maps will be nil maps, and as such, are not safe to assign to. (I consider this to be a design wart of the language.)