instanceof type guard missing deduction

TypeScript Version: 2.7.0-dev.201xxxxx

Code

class C { }

function f<T extends C>(v: T | string): void {
  if (v instanceof C) {
    const x: T = v;
    console.log(x);
  }
}

Expected behavior:

No compile errors:

Actual behavior:

p.ts(5,11): error TS2322: Type 'string | T' is not assignable to type 'T'.
Type 'string' is not assignable to type 'T'.

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