Allow Default Values for String Literal Types

[blink1073]

TypeScript Version:

1.8.2.

Code

// A self-contained demonstration of the problem follows...
class Foo {
    bar: "fizz" | "buzz";
}

class Bar extends Foo {
    bar = "fizz";
}

Expected behavior:
Bar.bar is initialized to "fizz";

Actual behavior:
Type 'string' is not assignable to type '"buzz"'

[blink1073]

This works:

interface Foo {
    bar: "fizz" | "buzz"
}

class Bar implements Foo {
    bar: "fizz" | "buzz" = "fizz";
}

[sccolbert]

Another example:

class Foo {
    bar: "fizz" | "buzz";
}

class Bar1 extends Foo {
    constructor() {
        super();
        this.bar = "fizz";  // okay
    }
}

class Bar2 extends Foo {
    bar = "fizz";  // fail
}

[DanielRosenwasser]

I think #6118 should fix this. CC @sandersn

[sandersn]

Or #10610

[blink1073]

I ran the original code through the 2.0.2 compiler, and the error remains.

[blink1073]

The message has changed to:

test.ts(6,7): error TS2415: Class 'Bar' incorrectly extends base class 'Foo'.
  Types of property 'bar' are incompatible.
    Type 'string' is not assignable to type '"fizz" | "buzz"'.

[sandersn]

Yep, neither #6118 or #10610 are merged yet. We are trying to determine how bad the breaking change is right now.

[blink1073]

Ah, fair enough.

[RyanCavanaugh]

Duplicate #10570