Index Signature Fails with Generics and Intersection Types

[AnyhowStep]

TypeScript Version: Version 2.9.0-dev.20180405

Search Terms: extends generic intersection index

Code

type SomeType<T> = {
    [k in keyof T]: any;
};

function generic<Base, Derived extends SomeType<Base>>() {
    const derived: Derived = null as any;
    const test: { test: string } = null as any;
    const both: Derived & { test : string } = null as any;

    const works0: SomeType<Base> = derived;
    const works1: SomeType<Derived> = derived;
    const works2: SomeType<{ test: string }> = test;

    //This fails unexpectedly    
    const fails0: SomeType<Base & { test: string }> = both;
}

interface ConcreteBase {
    a: string,
    b: string,
}
interface ConcreteDerived {
    a: string,
    b: string,
    c: string,
}
const derived: ConcreteDerived = null as any;
const test: { test: string } = null as any;
const both: ConcreteDerived & { test : string } = null as any;

const works0: SomeType<ConcreteBase> = derived;
const works1: SomeType<ConcreteDerived> = derived;
const works2: SomeType<{ test: string }> = test;

const worksOutFine: SomeType<ConcreteBase & { test: string }> = both;

Expected behavior:

fails0 in generic() should compile

Actual behavior:

fails0 in generic() fails

Playground Link: Link here

Intuitively, to me, it should work out fine.

• Derived has all the fields of Base; possibly more.
• I added one extra field test.
• Derived is assignable to Base
• Derived & { test: string } should be assignable to Base & { test: string }

[weswigham]

So the way we break down intersections makes it impossible to recognize the equivalence here. We compare the intersection one member at a time and see if either member on their own is assignable to the SomeType intersection. {test: string} isn't assignable - it's not guaranteed to have Base's props. Derived isn't assignable because it's missing test (keyof Base | "test" is not assignable to keyof Derived).

@ahejlsberg have we just been marking issues like this as a design limitation, or are we planning to do something about it?