Interface type coercion fails with unsatisfied properties in unions

🔎 Search Terms

interface type coercion
type coercion
unsatisfied union constraints
property missing in but required
conditional interference

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type coercion

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgILIN4Chm+QZ0gAcAuZARiwF8stRJZEUAhTHPQiU5AJmtvrR4SZMwD6AUQAeYZBBkQQAE3yi2eAsTJ8N8sFDj4yAIgBu0AJ7IEAexsAbY-yxgLRFAGViyALwVkAD68Lm4oAMI2IPgArgC20L5ogcgAFKxB4tJgAJQA3LS2UbIw0SAIiSnZZF5cvgB8yACycGAAFgB0Bso2sZXIDQAM7QCsyAD8vMhk5PmFhNaRMfFQFQDurS0Q5lBkEVFx0Nn1mMg0BYuynESJJWWV+VgA9I-IgKDkyKs2UADW+HQwKSuvh8fnIR2wGjmBygKQwmlqVGy1Dk9nwKAheChy1h8OuiOcz2QgBlyZDwYCorBY6A4oGIoA

💻 Code

interface A {
    step: 1
}

interface B {
    step: 2
}

interface B_Ext extends B {
    step: 2
    extras: "very cool"
}

type Step = 1 | 2
type Consumer = A | (B | B_Ext);

const func = (): Step => Math.random() > 0.5 ? 2 : 1;
const consumer = (whatever: Consumer) => { }


const step = func();

// ✅ works
if(step === 1) {
    consumer({ step })
} else {
    consumer({ step })
}

// ❌ fails
consumer({ step })

🙁 Actual behavior

The compiler expects that all properties of a sub-constituent be satisfied

🙂 Expected behavior

The union defines that it is either A | B and B might be B or B & Extras

Additional information about the issue

The same happens with types instead of interfaces as well as a the extras being passed as union of the type:

// causes the same issues
type A = { step: 1 }
type B = { step: 2 } & ({} | { extras: 'very good' })

Start by running the linked TypeScript Playground reproduction and compare the two accepted branch calls with the rejected standalone call. Trace the compiler's union assignability and interface/type coercion behavior; done means the standalone consumer({ step }) call is accepted without requiring extras, while the existing examples remain valid.