Nominal enum assignability after union

[ivogabe]

TypeScript Version: 2.0 RC. This was working correctly in 2.0 beta.

Code

module X {
    export enum Foo {
        A, B
    }
}
module Y {
    export enum Foo {
        A, B
    }
}
const x: X.Foo = Y.Foo.A;
const y: X.Foo | undefined = Y.Foo.A;

Expected behavior:
Both assignments are valid.

Actual behavior:
The first assignment is valid, the second throws an error saying "Type 'Foo' is not assignable to type 'Foo | undefined'."

This block a project that depends on the TypeScript api, and on some package that also depends on TypeScript. So TS is installed on node_modules/typescript and node_modules/foo/node_modules/typescript. (Normally NPM would install TypeScript only once, but foo is a symlink.) This causes that objects from one installation of TypeScript cannot be passed to the other. The following code illustrates that:

import * as ts from "typescript";
import * as ts2 from "foo/node_modules/typescript";

const identifier: ts.Identifier = undefined as any;
const identifier2: ts2.Identifier = identifier;

const ok: ts.SyntaxKind = ts2.SyntaxKind.AbstractKeyword;
const error: ts.SyntaxKind | undefined = ts2.SyntaxKind.AbstractKeyword;

Error on the declaration of identifier2:

Type 'ts.Identifier' is not assignable to type 'ts.Identifier'.
  Types of property 'originalKeywordKind' are incompatible.
    Type 'SyntaxKind | undefined' is not assignable to type 'SyntaxKind | undefined'.
      Type 'SyntaxKind.Unknown' is not assignable to type 'SyntaxKind | undefined'.

And on error:

Type 'SyntaxKind' is not assignable to type 'SyntaxKind | undefined'.

[RyanCavanaugh]

@mhegazy milestone?

[sandersn]

Note: requires "strictNullChecks": true to repro

[sandersn]

repro without strictNullChecks: const y: X.Foo | boolean = Y.Foo.A;

[sandersn]

The fix is very similar to the one for #10738, so I'm going to fix both in #10784.