[jakelazaroff]

That's basically case 1, right? The compiler knows which type is being used at each call site, so it can generate a separate function for each type in the union and eliminate the type check/dead branches.

I guess the exception would be if you have a non-homogenous array that you try to map over. In that case there's probably no way around boxing the values.

[xg15]

For consts and function arguments yes, I think. But you could have some mutable variable or field whose value depends on runtime state. In that case, you could have an actual polymorphic type.

[epolanski]

But the previous example isn't that case.

if (c.type === 'a') { /* c is of type A here */ }

This is dynamic typing, this code is checked at runtime, and it's leveraged statically.

[xg15]

Yes, that's correct. In the GP example, c was const, so the type can be determined at compile time.

[simulate-me]

I was writing TypeScript, where const just means the variable is not reassigned. For instance this is valid:

    const c: C = Math.random() < 0.5 ? { type: 'a' } : { type: 'b' }.

[xg15]

Ah, I'm sorry. You're right of course!