Issue implementing interface

[testerez]

TypeScript Version: 2.2.2

Code

interface I {
  foo: 'foo' | 'bar';
}

interface IClass {
  i: I;
}

class C implements IClass {
  i = { foo: 'foo' };
}

Expected behavior:
no error

Actual behavior:

Class 'C' incorrectly implements interface 'IClass'.
Types of property 'i' are incompatible.
Type '{ foo: string; }' is not assignable to type 'I'.
Types of property 'foo' are incompatible.
Type 'string' is not assignable to type '"foo" | "bar"'.

[ghost]

The type of 'foo' will be widened to string in this case. You can fix this by adding a type annotation: i: I = { foo: 'foo' };.

[vkurchatkin]

Typescript is unable to infer literal types, unless it is a constant. For example:

let x = 1;
const y: 1 = x; // error, but shouldn't be

const x = 1;
const y: 1 = x; // no error

const x = { foo: 'bar' };
const y: { foo: 'bar' } = x; // error, but shouldn't be

const x: { foo: 'bar' } = { foo: 'bar' };
const y: { foo: 'bar' } = x; // no error

[testerez]

@Andy-MS yes I did something similar: i = { foo: 'foo' } as I;
But It seams like typescript could do better here...

@vkurchatkin in your third example, x is constant but not x.foo...

[vkurchatkin]

@testerez right, you can't make property a constant in TypeScript. Still, this shouldn't be an error.

[RyanCavanaugh]

Duplicate #7011 - if the initializer were contextually typed this would work as expected