Uninitialized local check still permits provably erroneous use-before-assign in closure

🔎 Search Terms

uninitialized, local, use before assign, control flow

🕗 Version & Regression Information

• This changed in commit or PR https://github.com/microsoft/TypeScript/pull/55887

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240925#code/DYUwLgBAZg9jBcEDOYBOBLAdgcwNwCh8BjGTFZcAMTggF4IAKASkQDcZ0ATOgPggG98ECOiiNYMAHSgcYABYQAPBACMTAUOHQa9AOQTdBYQF98xgvhAAPAA4xUkfqaA

💻 Code

let foo: string;

const setFoo = (): void => {
  if (foo.length < 1) {
    foo = 'foo';
  }
};

export {}

🙁 Actual behavior

No TS Error

🙂 Expected behavior

Variable 'foo' is used before being assigned.(2454)

Additional information about the issue

(this error is correctly reported if we never assign at all, as below)

let foo: string;

const setFoo = (): void => {
  if (foo.length < 1) {
  }
};

export {}

Related, https://github.com/microsoft/TypeScript/issues/60064.

Otherwise this can be solved by not considering assignments which occur unconditionally after a read.

Start with the linked TypeScript Playground reproduction and compare it with the additional example that already reports diagnostic 2454. Read the control-flow change in PR 55887 and related issue 60064; done means the closure read reports use-before-assign without breaking the existing no-assignment diagnostic behavior.