Return type of generic function interface allows object literals to have unknown properties.

[shaunluttin]

TypeScript Version: 2.4.0

Code

type MyState = { foo: number; };
type Reducer<T> = (state: T) => T;

const reducer1: Reducer<MyState> = (state) => {
  return { foo: 123, bar: 123 }; // works.
};

const reducer2 = (state: MyState) : MyState  => {
  return { foo: 123, bar: 123 }; // fails.
  
  // Type '{ foo: number; bar: number; }' is not 
  // assignable to type 'MyState'.
  // Object literal may only specify known properties, 
  // and 'bar' does not exist in type 'MyState'.
};

Expected behavior:

The implementations for reducer1 and reducer2 have the same behavior.

Actual behavior:

The implementations for reducer1 and reducer2 have different behavior.

[mhegazy]

Please see #7547