Can't cast array to array subtype

TypeScript Version: 2.7.0-dev.20171109

Code

interface I extends Array<number> { x: number }
const x = [] as I;

Expected behavior:

No error.

Actual behavior:

src/a.ts(2,11): error TS2352: Type 'never[]' cannot be converted to type 'I'.
  Property 'x' is missing in type 'never[]'.

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