Using "implements" causes an issue with string literal types. "extends" does not.

[dsifford]

TypeScript Version: nightly (2.0.0-dev.201xxxxx)

Relevant StackOverflow discussion

Playground

Kudos to @nitzantomer for looking into this

Code

// This is defined in a d.ts file. 
class Test {
    someObj: {
        someString: 'this'|'that'|'the other'
    };
}

// This is me actually using the class.
class Test2 implements Test {
    someObj = {
        someString: 'this'
    }
}

Expected behavior:
No error thrown. Using literal 'this' as the value for someObj.someString correctly implements class.

Actual behavior:
Error thrown.

Class 'Test2' incorrectly implements interface 'Test'.
  Types of property 'someObj' are incompatible.
    Type '{ someString: string; }' is not assignable to type '{ someString: "this" | "that" | "the other"; }'.
      Types of property 'someString' are incompatible.
        Type 'string' is not assignable to type '"this" | "that" | "the other"'.
          Type 'string' is not assignable to type '"the other"'.

[weswigham]

@sandersn Was looking into a related issue before - contextually typing inherited methods - I believe this is similar in nature.

[DanielRosenwasser]

Duplicate of #1373 and #3667.

See @sandersn's PR and explanation of why we couldn't do this at #6118 (comment).