Error in a case without type annotation.

[lacolaco]

Playground

interface PolymerElement {
  properties?: { [propName: string]: StringConstructor|PolymerElementProperty };
}

interface PolymerElementProperty {
  type: StringConstructor;
}

// Why is this incorrectly implements?
class WithoutTypeAnnotation implements PolymerElement {
    properties = {
        prop1: String,
        prop2: {
            type: String
        }
    };
}

class WithTypeAnnotation implements PolymerElement {
    properties:{ [propName: string]: StringConstructor|PolymerElementProperty } = {
        prop1: String,
        prop2: {
            type: String
        }
    };
}

Both properties are same object, but tsc occurs error in a case without type annotation. Why?

Thanks.

[RyanCavanaugh]

Necessary background: http://stackoverflow.com/questions/22077023/why-cant-i-indirectly-return-an-object-literal-to-satisfy-an-index-signature-re

The root problem here is that we don't contextually type class property initializers with the type the property "should" have from an implements or extends clause. Without a contextual type on the object literal, the property is inferred to not have an index signature.

Logged #3667 as this has been a long-standing pain point.

[lacolaco]

Thanks, Ryan. I understood the problem's background.
I hope it'll be resolved someday.