`export let` variable doesn't require the variable to be assigned within current file

🔎 Search Terms

export let, undefined, assignment, assigned

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about export let

⏯ Playground Link

No response

💻 Code

// a.ts
export let x: { foo(): void }

if (!!false) {
    // @ts-expect-error Variable 'x' is used before being assigned.
    x.foo()
}

// b.ts
import { x } from './a.ts'

// TS compiles, throws at runtime
x.foo()

🙁 Actual behavior

TS compiles, only throws at runtime.

🙂 Expected behavior

TS refuses to compile without ! non-null assertion, with error probably being reported in the exporting module a.ts rather than the importing module b.ts.

Additional information about the issue

No response

Start with the cross-file example in a.ts and b.ts and reproduce the current compilation and runtime behavior. Trace the definite-assignment check for the exported x and its use after import; done means the compiler reports the missing assignment rather than allowing the runtime failure.