Unsoundly loses one branch when another is uninitialized, at top of loop

[gnprice]

Flow version: v0.137.0 (and older versions on flow.org/try, including at least v0.37.0 which is the oldest)

Expected behavior

Should report an error in the following code, which fails at runtime with TypeError:

let thing;
for (const x of [0, 1]) {
  if (thing) {
    thing.nonsense(); // boom! but no error
  }
  thing = { x: 1 };
}

Actual behavior

No error is reported.

On inspection with an IDE, flow type-at-pos, or $Flow$DebugPrint, Flow is apparently seeing the variable's type where it appears at the top of the loop as simply void, when it should be void | {| x: number |}. That'd be enough to cause this issue.

A key element of the repro case is that on entering the loop, the variable is uninitialized. If instead the variable is initialized with undefined -- which should have the exact same behavior at runtime -- then Flow correctly reports an error:

let other = undefined;
for (const x of [0, 1]) {
  if (other) {
    other.nonsense(); // error, correctly
  }
  other = { x: 1 };
}

and inspection shows it correctly sees the variable's type as void | {| x: number |}.

• Link to Try-Flow: here

[gnprice]

Here's another bit of unsoundness involving uninitialized variables which is sort of an inverse to this one; not sure if it's the same underlying issue.

If instead of using, and then initializing, the variable in a loop body, we do the same thing in a function expression's body (perhaps because we're expressing the same loop using Array.prototype.forEach)... then instead of forgetting that the variable might be already initialized, Flow forgets that it might still not yet be initialized:

let thing;
[0].forEach(() => {
  thing.f(); // boom! but no error
  thing = { f: () => {} };
})

Inspection shows that Flow sees the variable's type as the type that the code further down would initialize it to.

Here again, if the variable is explicitly initialized to undefined, Flow sees both possibilities and correctly reports an error:

let other = undefined;
[0].forEach(() => {
  other.f(); // error, correctly
  other = { f: () => {} };
})

And here's a Try-Flow link for these.

[SamChou19815]

This is now fixed.