Strict null assertion error on an already-defined object property

[an5rag]

TypeScript Version: 2.8.1

Code

// Compile with --strict
interface IFoo {
 a?: number[],
}

let foo: IFoo = {
 a: []
};

// error: Object is possible 'undefined'
foo.a.push(4);

// this will work
// foo.a = [];
// foo.a.push(5) // no errors
// alternatively,
foo!.a.push(3);

Expected behavior:
The compiler should be able to infer using the object initialization that foo.a is defined.

Actual behavior:
The compiler ignores the initialization of property 'a'.

Playground Link: Playground link (use strict check in options)

Related Issues:

[mhegazy]

Looks like a duplicate of #20219, should be fixed by #22006