Union type of a interface with array of the same interface fails to check interface optional properties

[matthauck]

TypeScript Version:

1.8.10

Code

interface SomeInterface {
    name?: string;
}

// these two (correctly) produce compiler errors
//var broken1: SomeInterface = { another: 'a' };
//var broken2: SomeInterface[] = { another: 'b' };

// this does not produce a compiler error but should
var broken3: SomeInterface | SomeInterface[] = { another: 'c' };

Expected behavior:
Compilation should fail

Actual behavior:
Compilation succeeds

Note: This behavior only seems to manifest when the interface contains only optional properties. If I change name to be a required property then compilation fails with:

test.ts(10,5): error TS2322: Type '{ [x: number]: undefined; another: string; }' is not assignable to type 'SomeInterface | SomeInterface[]'.
  Type '{ [x: number]: undefined; another: string; }' is not assignable to type 'SomeInterface[]'.
    Property 'length' is missing in type '{ [x: number]: undefined; another: string; }'.

[RyanCavanaugh]

@ahejlsberg seems like this should have been fixed when we fixed #5237 ?

[ahejlsberg]

We currently do the excess property check only if no constituent of the target has an index signature (of either kind). An array has only a numeric index signature and we probably should factor that into our reasoning. Specifically, we should say that if some constituent of the target has a numeric index signature, then only numerically named properties are expected.

[sandersn]

The fix is up at #8822.

[matthauck]

Thanks! 🎉