Repository: TypeScript
Issue: Type Alias Record/Interface Selector Pattern - Discriminated Unions, Fails to discriminate type shape, like switch does
Labels: Suggestion,Awaiting More Feedback

It would be great, if this pattern would work, because it is so much simpler to use and would be so much faster typically than extends ? pattern, because there is no interrogation of the types with all complex patterns and shapes and forms, which gets quite hairy, performance slow downs major. With this pattern it just becomes iterative selection, hopefully just dictionary as is javascript in the backing the code. It would also ensure that everything is typechecked correctly for all of these interesting ways to use typescript.

typescript@3.3.0-dev.20181128 

```.ts
type TSPrimatives = boolean | number | string | Date;

type TSShapes = ShapeTsType<any> | ShapeTsRecord<any> | ShapeTsArrayPrimative<any> | ShapeTsArrayRecord<any>;

interface ShapeTsType<T extends TSPrimatives>
{
    __Type : T
    __Shape: 'T'
}

interface ShapeTsRecord<T extends Record<string, TSShapes>>
{
    __Record : T
    __Shape: 'R'
}

interface ShapeTsArrayPrimative<T extends TSShapes>
{
    __ArrayPrimative : {w:T}
    __Shape:'AP'
}

interface ShapeTsArrayRecord<T extends Record<string,TSShapes>>
{
    __ArrayRecord : T
    __Shape:'AR'
}

type ExtractShape<T extends ShapeTsType<any>> = T['__Type']

type ExractTsShape<T extends TSShapes> = ({
    'T' : ExtractShape<T> // Fails to discriminate the type
    'R' : T['__Record'] // Fails to discriminate the type
    'AP' : T['__ArrayPrimative']  // Fails to discriminate the type
    'AR' : T['__ArrayRecord']  // Fails to discriminate the type
})[T['__Shape']]

type ExtractRecordTSShape<T extends Record<string, TSShapes>> = 
{
    [K in keyof T] : ExractTsShape<T[K]>
}
```

https://www.typescriptlang.org/docs/handbook/advanced-types.html

Discriminated Unions
You can combine singleton types, union types, type guards, and type aliases to build an advanced pattern called discriminated unions, also known as tagged unions or algebraic data types. Discriminated unions are useful in functional programming. Some languages automatically discriminate unions for you; TypeScript instead builds on JavaScript patterns as they exist today. There are three ingredients:

Types that have a common, singleton type property — the discriminant.
A type alias that takes the union of those types — the union.
Type guards on the common property.
interface Square {
    kind: "square";
    size: number;
}
interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}
interface Circle {
    kind: "circle";
    radius: number;
}
First we declare the interfaces we will union. Each interface has a kind property with a different string literal type. The kind property is called the discriminant or tag. The other properties are specific to each interface. Notice that the interfaces are currently unrelated. Let’s put them into a union:

type Shape = Square | Rectangle | Circle;
Now let’s use the discriminated union:

function area(s: Shape) {
    switch (s.kind) {
        case "square": return s.size * s.size;
        case "rectangle": return s.height * s.width;
        case "circle": return Math.PI * s.radius ** 2;
    }
}