Generic function does not accept argument of type with union and intersection

[turtleflyer]

Bug Report

🔎 Search Terms

generic function union intersection type optional property

🕗 Version & Regression Information

• This changed between versions 3.6.0-dev.20190723 and version 3.6.0-dev.20190724

⏯ Playground Link

Playground link with relevant code

💻 Code

type A = ({ a: any } | { b: any }) & { c?: any };

declare function testA<Param extends A>(param: Param & A): Param;

declare const s1: A;

const a1 = testA(s1);
/**
 * Argument of type 'A' is not assignable to parameter of type '{ b: any; } & A'.
 *   Type '{ a: any; } & { c?: any; }' is not assignable to type '{ b: any; } & A'.
 *     Type '{ a: any; } & { c?: any; }' is not assignable to type '{ b: any; } & { a: any; } & { c?: any; }'.
 *       Property 'b' is missing in type '{ a: any; } & { c?: any; }' but required in type '{ b: any; }'.(2345)
 */

type B = ({ a: any } | { b: any }) & { c: any };

declare const s2: B;

const a2 = testA(s2); // Type of result is A but must be B

🙁 Actual behavior

The line const a1 = testA(s1); gives an error:

Argument of type 'A' is not assignable to parameter of type '{ b: any; } & A'.
  Type '{ a: any; } & { c?: any; }' is not assignable to type '{ b: any; } & A'.
    Type '{ a: any; } & { c?: any; }' is not assignable to type '{ b: any; } & { a: any; } & { c?: any; }'.
      Property 'b' is missing in type '{ a: any; } & { c?: any; }' but required in type '{ b: any; }'.(2345)

In the line const a2 = testA(s2); the typescript compiler infers the constanta a2 has type A

🙂 Expected behavior

The variable s1 has the type A that means function testA<Param extends A>(param: Param & A): Param accepts s1. The parameter type Param is instantiated to A.

The constant a2 has the type B since the parameter s2 has the type B.

It was a normal behavior up to version 3.6.0-dev.20190723. Since then it leads to errors.

[turtleflyer]

Also, it is worth to note that the compiler made the wrong conclusion in the error message when it reveals { b: any } & A as { b: any; } & { a: any; } & { c?: any; }

[RyanCavanaugh]

What was the intent in writing

declare function testA<Param extends A>(param: Param & A): Param;

instead of

declare function testA<Param extends A>(param: Param): Param;

?

[turtleflyer]

This was an example that simplifies the real-life piece of code exposing the bug. This pattern is useful in a generic function where types are not narrowing properly due to design limitation:

type Animal = { type: 'dog'; forelegs: number } | { type: 'duck'; wings: number };

function calculateForelimbs<A extends Animal>(animal: A): A & { forelimbs: number } {
    return { ...animal, forelimbs: animal.type === 'dog' ? animal.forelegs : animal.wings };
    // won't work due to design limitation
}

It could be fixed by adding & Animal to the type of animal argument:

function calculateForelimbsWorkable<A extends Animal>(animal: A & Animal): A & { forelimbs: number } {
    return { ...animal, forelimbs: animal.type === 'dog' ? animal.forelegs : animal.wings };
    // working like a charm
}

function createSuperAnimal(): Animal {
    const legsOrWings = 100500;
    const typeOfAnimal = Math.random() < 0.5 ? 'dog' : 'duck';

    return typeOfAnimal === 'dog'
        ? { type: typeOfAnimal, forelegs: legsOrWings }
        : { type: typeOfAnimal, wings: legsOrWings }
}

const superAnimalWithLimbsCalculated = calculateForelimbsWorkable(createSuperAnimal());

link to playground

If we make the Animal type more complex:

type Animal = ({ type: 'dog'; forelegs: number } | { type: 'duck'; wings: number }) & { domesticated: boolean };

it still works (link to playground).

When we decide to have property domesticated being optional, it turns out the code is broke:

type Animal = ({ type: 'dog'; forelegs: number } | { type: 'duck'; wings: number }) & { domesticated?: boolean };

link to playground

Now, when version 4.3.2 with improved contextual narrowing for generics has been shipped, it has become less actual. Now we can skip & Animal part.

But the bug is a bug. It may affect the legacy code or be a broken part of a bigger problem.

[turtleflyer]

I found the use case where this pattern would be still useful:

type A = ({ a: any } | { b: any }) & { c?: any };

declare function f<X extends A>(x: X): X

function testA1<Param extends A | undefined>(param: Param): Param {
    return param && f(param)
    // won't work even in typescript 4.3
};

function testA2<Param extends A | undefined>(param: Param & A): Param {
    return param && f(param)
    // working fine
};

declare const s1: A;

const a1 = testA2(s1); // still error

type B = ({ a: any } | { b: any }) & { c: any };

declare const s2: B;

const a2 = testA2(s2); // wrong type A | undefined, must be B

const a3 = testA1(s1); // A, but testA1 would not work on its own

const a4 = testA1(s2); // B, but testA1 would not work on its own

link to playground

It works fine in typescript 3.3.3333: link