Type assignment between object types fails even when compatibility was checked through type-guards on properties

[fpascutti]

From section 3.11.4 (assignment compatibility) of the specification, an object type T should be assignable from an object type S when all T's properties exist on type S (same name) and each can be assigned from the corresponding property on type S.
However this does not work when property assignability was proven through a type-guard.

TypeScript Version: 2.5.2

Code

interface Widget {
  readonly data: string | number;
}
interface Gadget {
  readonly data: string;
}

function foo(g: Gadget): void {
  console.log(g.data);
}
function bar(w: Widget): void {
  if (typeof w.data === "string") {
    const d: string = w.data; // `w.data` correctly recognized as a `string`
    foo(w); // however `w` cannot be used as a `Gadget`
  }
}

Expected behavior:

Compiles successfully.

Actual behavior:

Compilation fails with:

error TS2345: Argument of type 'Widget' is not assignable to parameter of type 'Gadget'.
  Types of property 'data' are incompatible.
    Type 'string | number' is not assignable to type 'string'.
      Type 'number' is not assignable to type 'string'.

[RyanCavanaugh]

See #10065

[fpascutti]

Thanks. I did not find this issue.
Your latest comment makes sense by the way.