TypeScript Version: 2.2.0-dev.20170130 (npm version) / master

Code

interface Bar {  bar: number;  foo: object | null;  }

let a = {
  test<K extends keyof Bar> (a: K,  b?: Bar[K]  |  null)  {
    console.log(a, b);
  }
};
a.test("bar", null);

Expected behavior:
tsc --strictNullChecks compiles it without errors.

Actual behavior:
It reports:

error TS2345: Argument of type 'null' is not assignable to parameter of type 'number | undefin
ed'.

Notes
This case only occurs if at least one member of Bar can be null and --strictNullChecks is enabled.

I think the checker should not merge null and undefined in optional union-type args.