Type should be simplified for quickInfo

[ghost]

TypeScript Version: nightly (2.4.0-dev.20170609)

Code

/// <reference path="fourslash.ts" />
////interface I { x: string | number; }
////declare const i: I & { x: number };
////i./**/x;

verify.quickInfoAt("", "(property) x: number");

Expected behavior:

Test succeeds.

Actual behavior:

Shows (property) x: number | (string & number).

[mhegazy]

not sure what you mean by "simplified". that s the type of x.

[ghost]

When we intersect { x: number } with { x: string | number }, can't we simplify it to just { x: number }?

[mhegazy]

nope... it is not just a number, it is number & (string | number) which then distributes to (number & string) | (number & number) which is (number & string) | number.

it is true that number & string is an empty set, but we have no special treatment for these in intersection type.
you could argue that the compiler should reduce types that are guaranteed to have no values to never, but that would break cases like tagged types (e.g. string & {tag: any}). we do use that for instance see Path in src\compiler\types.ts

[jcalz]

@mhegazy Even if (number & string) weren't empty, number & (string | number) should still reduce to number by absorption (see coincidentally recent #16386).

[mhegazy]

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