Weird uninitialized variable error for destructured members of type parameter constrained to homomorphic mapped type where member is declared optional

[DanielRosenwasser]

Found in Formik by Wesley Wigham (@weswigham) who patiently worked with me to find the minimal repro.

Run the following with strictNullChecks

interface Options {
 a?: number | object;
 b: () => void;
}

class C<T extends Options> {
 foo!: { [P in keyof T]: T[P] }

 method() {
 let { a, b } = this.foo;
 !(a && b)
 a
 // TypeScript issues an error on 'a' above:
 // Variable 'a' is used before being assigned.'
 }
}

[rozzzly]

Daniel Rosenwasser (@DanielRosenwasser)

 foo!: { [P in keyof T]: T[P] }

Is the !: some new/obscure syntax I'm not aware of? Or just a typo?

[ahejlsberg]

Patrick Lienau (@rozzzly) See #20166.

[sandersn]

Should be fixed #20995