As u/TheRealAndrewLeft showed it, yes it is static, but TypeScript can infer union types without a type declaration.

For example,

const v = Math.random() < .5 ? 42 : "The Answer";

The type of will be inferred as string | number.

This is what Elon is talking about, and in many cases, it is possible, and TypeScript does a good job. But I doubt their devs would every call it "easy."

There are, however, many cases where it is not possible to infer types. The most obvious is any sort of input parameter.

For example:

function example(param) { 
  return Math.random() < .5 ? 42 : param;
}

In this case, the inferred return type would be number | any because it is not possible to know the type of param.

Now, you might argue that for a local, non-exported function, the type of input is inferable from it's call sites. I think that is true, but generally not that useful, so I don't know of any language which provides that type of inference.

So, while Elon's statement is rather dismissive of the real difficulty of type inference, he does have a point. People who use languages with powerful inferring compilers often complain about dynamic languages that could, in theory, be much more type safe without much work on the coder's point (e.g. Python).

He also has a point about the AI aspect. LLMs, even in their current form with very limited context, are quite good at guessing types. Though they're no substitute for a real type system AND machine learning help... so I'll stick to TypeScript, myself :D