The behavior of the intersection type of array changed in 2.6.1

[icepeng]

TypeScript Version: 2.6.1

Code

interface A {
 first: string;
 bList?: B[];
}

interface B {
 second: string;
 c?: C;
}

interface C {
 third: string;
}

const Q: A & { bList: (B & { c: C })[] } = {
 first: 'first',
 bList: [
 {
 second: 'second',
 c: {
 third: 'third',
 },
 },
 ],
};

Q.bList.map(item => {
 P(item);
});

function P(value: B & { c: C }) {
 console.log(value);
}

Expected behavior:
Until 2.5.1 this code did not generate a compilation error.
The type of bList is calculated as follows:

(undefined & (B & {
 c: C;
})[]) | (B[] & (B & {
 c: C;
})[])

And type of item

B & {
 c: C;
}

Actual behavior:
With 2.6.x, this code causes an error.
The type of bList is calculated as follows:

B[] & (B & {
 c: C[];
})[]

And type of item

B

The type of item is treated as B and an error occurs. Is this change specified in the changelog?

[ahejlsberg]

This is an effect of #18438. We now remove empty intersection types (i.e. intersection types for which no values exist) from union types. In your example, this means that undefined & (B & { c: C })[] is removed when constructing the type for bList, leaving just B[] & (B & { c: C })[]. Then, as we look for a map member on that type, we end up picking the candidate from B[] and therefore item has type B. Previously we'd pick a candidate from (B & { c: C })[] because we didn't remove the empty intersection.

In general, when the constituent types of intersections have overlapping signatures, the intersection is order sensitive because overload resolution processes the signatures from left to right. Indeed, your example works if you change the type of Q to list A last:

const Q: { bList: (B & { c: C })[] } & A = { ... }; // Now works 

[icepeng]

It would be nice to write that the intersection type is order sensitive in the advanced types section of the typescript handbook.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.