Nullish coalescing assignment operator does not narrow correctly

[danielrentz]

TypeScript Version: 4.0.2

Search Terms: nullish coalescing possibly undefined

Code

let x: string | undefined;

let a: string | undefined;
a ??= "x";
a.length;

let b: string | undefined;
b ??= x;
b.length;

let c: string | undefined;
c = c ?? x ?? "x";
c.length;

let d: string | undefined;
d ??= x ?? "x";
d.length;

Expected behavior:
a.length passes, b.length fails (b undefined), c.length passes, d.length passes, because it becomes a string

Actual behavior:
d.length fails because not narrowed to string.

Playground Link: https://www.typescriptlang.org/play?#code/DYUwLgBAHgXBDOYBOBLAdgcwgHwgVzQBMQAzdEQgbgChrRIBDORVTHfI08q6hiAfn4BeCACIoomgwB0oTGAAWNOuAgAjZsnRZcBYmTQUaagcOjHZIeUtr0IAY02sdHfdxr2IIz4OimxEh6W1sp2hE7a7HpchjyEpiJQ-uKS1ITBGIo0QA

Related Issues: none found

[hopha95]

I'd like to take a look into this bug :)

[danielrentz]

Btw, same problem arises with the ||= operator.

[Kingwl]

Seems caused by control flow bind.

Current flow is

 Branch ┬ True (d) ───────────────────────────────╮             ──────────┬ Start  
        ├ True (x) ───────────────────────────────╫───────────┬ False (d) ╯   
        ├──────────────────────────                 False (x) ╯               
        ├ True (d ??= x ?? "x") ─┬ Assignment (d) ╯                           
        ╰ False (d ??= x ?? "x") ╯ 

The True(x) branch has been ignored.

[eventualbuddha]

I independently ran into this bug and confirmed that my reduced test case fails in 4.1.4 but passes with the playground for #40536:

let a: number | undefined
let b: number | undefined

// TS fails to infer that `a` must be `number`, so `a + 1` errors with:
// Object is possibly 'undefined'. (2532)
a ??= b ?? 1
console.log(a + 1)

// Should be equivalent but without the assignment to `a`, but this works.
console.log((a ?? b ?? 1) + 1)

Is there any reason that fix can't be merged yet?