Functions passed as an argument to other functions are not type checked as expected

[michaelmesser]

I expected the following code to produce an error as function a could pass an argument that is not a string to function b.

Code:

declare function a(f: (v: Object) => string): void
declare function b(f: string): string
a(b)

Expected Result:

error TS2345: Argument of type '(f: string) => string' is not assignable to parameter of type '(v: Object) => string'.
  Types of parameters 'f' and 'v' are incompatible.
    Type 'Object' is not assignable to type 'string'.

Actual Result:
No error

[RyanCavanaugh]

See e.e.g #6102