Conditional type based on enum is not respected when branches include class instances with different constructor signatures

[cyorobert]

TypeScript Version: 3.7.x-dev.201xxxxx

Reproduced Issue with TS Nightly: http://www.typescriptlang.org/play/?ts=3.8.0-dev.20191125#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-NF0B1hxIJv4YjkCjt2n486Xa5QPsAqmZfgJgD066gJB8HG4UEUcG8HCRDygyUpaYz59CHB8SZ0UBSmxBSTJ4m4t93FKEUECQhKT8cigIRMjgDhhzAEAODGNs0AEKCkhgikyg9V41Q+atBxLEdEF0SxzUQex1DsKBU3TOBM2zRxc21AFPGCbgCQIxxdhFAERC8cQLCbFslzdIkSTJCkqRpTVu2uSxGULVl7nEOABCgbI+E0cpATGV9Jk2DgyhAHJ30Zax2MiTi+VwsdCFISh+IEyIhK9USZx+OcZUkgBGAAmABmIy41cNAHn3c0oHLKBUO4dD3kUSweNwSzJyMoA

Search Terms:

• typescript conditional type class instances
• typescript conditional type with different constructors signatures
• typescript conditional type with enum example

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
enum Food {
   SANDWICH,
   SOUP
}
interface MealPrep<FoodType> {
   makeFood(): FoodType
}
class Sandwich {
    constructor(private readonly ingredients: string[]) { }
}
class Soup {
   constructor(private readonly ingredients: string[]) { }
}
class SandwichService implements MealPrep<Sandwich> {
    constructor(private readonly foo: number) {}
    makeFood(): Sandwich {
        return new Sandwich(['ham', 'cheese']);
    }

}
class SoupService implements MealPrep<Soup> {
    constructor(private readonly foo: string) {}
    makeFood(): Soup {
        return new Soup(['tomato', 'crouton']);
    }
}

type NarrowedFood<T extends Food> =
    T extends Food.SOUP ? SoupService :
    T extends Food.SANDWICH ? SandwichService :
    never

// Is it possible to narrow this type or structure this in a way that avoids unions?
function getMealPrep<T extends Food>(food: T): NarrowedFood<T> {
    switch (food) {
        case Food.SOUP:
            return new SoupService('example of varying constructor signature')
        case Food.SANDWICH:
            return new SandwichService(123)
    }
}

const example = getMealPrep(Food.SANDWICH)
const sandwich = example.makeFood() // does correctly resolve to Sandwich

I would like to use an enum to construct one of a fixed set of types and then use a conditional type to narrow the return value to a single type that's relevant for a given branch of an enum.

In the example above, if I supply getMealPrep(Food.SOUP) I would like the inference engine to understand that I will get back a SoupService instance. This works as expected when SoupService and SandwichService have the same constructor signatures, however, when the signatures of SoupService and SandwichService differ I get the error Returned expression type SoupService is not assignable to type NarrowedFood<T>.

Expected behavior:
I would expect not to receive an error in getMealPrep when SoupService and SandwichService have different constructor type signatures.

Actual behavior:

Playground Link:

http://www.typescriptlang.org/play/?ts=3.8.0-dev.20191125#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-NF0B1hxIJv4YjkCjt2n486Xa5QPsAqmZfgJgD066gJB8HG4UEUcG8HCRDygyUpaYz59CHB8SZ0UBSmxBSTJ4m4t93FKEUECQhKT8cigIRMjgDhhzAEAODGNs0AEKCkhgikyg9V41Q+atBxLEdEF0SxzUQex1DsKBU3TOBM2zRxc21AFPGCbgCQIxxdhFAERC8cQLCbFslzdIkSTJCkqRpTVu2uSxGULVl7nEOABCgbI+E0cpATGV9Jk2DgyhAHJ30Zax2MiTi+VwsdCFISh+IEyIhK9USZx+OcZUkgBGAAmABmIy41cNAHn3c0oHLKBUO4dD3kUSweNwSzJyMoA

Related Issues:
I couldn't find any bugs that looked similar.

[RyanCavanaugh]

See #33912

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.