Mapped generic types lose index signature

[tyrielv]

TypeScript Version: 2.9.0-dev.20180402

Search Terms: partial mapped types index signature enum

Code

enum Fruits {
    Mango = 0,
    Banana = 1
}

interface IFruitMetadata<T> {
    [Fruits.Mango]: T;
    [Fruits.Banana]: T;
}

const onSale1: IFruitMetadata<boolean> = {
    [Fruits.Mango]: true,
    [Fruits.Banana]: false
};

const onSale2: Partial<IFruitMetadata<boolean>> = {
    [Fruits.Banana]: true,
};

const onSale3: Pick<IFruitMetadata<boolean>, Fruits.Banana> = {
    [Fruits.Banana]: true,
};

onSale1[Fruits.Banana] = true;
onSale2[Fruits.Mango] = true;
onSale3[Fruits.Banana] = true;

Expected behavior:
No compile errors. This was the behavior in 2.7.2.
Actual behavior:
With noImplicitAny=true, the Partial and Pick versions of the type triggers error TS7017: Element implicitly has an 'any' type because type 'Partial<IFruitMetadata>' has no index signature.
Playground Link

#22892 looks like it might be related.

[mhegazy]

@weswigham this should be fixed by #23090, correct?

[weswigham]

I think so? But @ahejlsberg was unenthused with that fix and was looking for a more holistic approach to the problems and inconsistencies we have in the keyof area in general. (Though I may have a revision that's a little closer to acceptable)

[mhegazy]

Spoke to @ahejlsberg, he has a different proposal for adding a new indexof operator. should address both issues.

[dardino]

Another example:

enum MyEnum {
    ValA = 0,
    ValB = 1,
}

type IEnumTpProp<R> = {[key in keyof typeof MyEnum]: R };

var X: IEnumTpProp<string> = {
    ValA: "text1",
    ValB: "text2",
    0: "error" // expected: error;  current: error; //OK
};

X[MyEnum[MyEnum.ValA]] = "no error"; // current: no error; expected: no error // **OK**
X[MyEnum.ValA] = "no error??"; // current: no error; expected: error // **KO**

[mhegazy]

Sample described in OP should be fixed by #23592.

@dardino the behavior in your example is expected, since enums have a numeric index signature.
with #23592 you can write something like:

enum MyEnum {
    ValA = 0,
    ValB = 1,
}

type IEnumTpProp<R> = { [key in MyEnum]: R };

var X: IEnumTpProp<string> = {
    [MyEnum.ValA]: "text1",
    [MyEnum.ValB]: "text2",
    0 :""  //Error, duplicate entry
};