Weak type detection for intersection types

[AlexeyMz]

TypeScript Version: 2.6.0-dev.20171018

Code

interface X { x: string }
interface Y { y: number }
interface Z { z?: boolean }

type XY = X & Y;
const xy: XY = {x: 'x', y: 10};

const z1: Z = xy;
// (1) compiles without any errors

const z2: Z = xy as Pick<XY, keyof XY>;
// (2) produces error: Type 'Pick<X & Y, "x" | "y">' has no properties in common with type 'Z'.

Expected behavior:
(1) should produce an error similar to (2) because there are no common properties for XY and Z types.

Actual behavior:
(1) compiles without any errors.

[sandersn]

I looked at the code and I think the exclusion of intersection sources is a side-effect of the original implementation, which only worked on object types. But it's trivial to allow intersections as source types.

I didn't see any new errors on the user tests, and on Definitely Typed, only in react-native-sortable-list and react-native-swiper, where we now detect the assignment of a branded number to an object type. The branded number came from a mapped type, so this is a subtle bug, and a very good catch.

I'll make a test and send out a PR.