And expression inferrs wrong type.

[dbaeumer]

TypeScript Version: 3.1.0-dev.201xxxxx

Search Terms:

Code

let b: boolean;
let s: string;

let x = b && s;

Expected behavior:

The inferred type of x is boolean | string;

Actual behavior:

The inferred type of x is string;

Playground Link:

https://www.typescriptlang.org/play/#src=let%20b%3A%20boolean%3B%0D%0Alet%20s%3A%20string%3B%0D%0A%0D%0Alet%20x%20%3D%20b%20%26%26%20s%3B

Related Issues:

[AlCalzone]

Since the type of x seems to take the value of b into account, should the type even be string?

let b: boolean;
let s: string;

let x1 = b && s; // string, (which should it be? x1 is undefined)

b = true;
let x2 = b && s; // string

b = false;
let x3 = b && s; // boolean

[markusjohnsson]

With strictNullChecks, you will get string | boolean.

[RyanCavanaugh]

@ahejlsberg in non-SNC we should really be keeping the left operand type in the result when it's a primitive

[ahejlsberg]

We've discussed fixing this before, but decided against it because of the breaking change. Do we feel differently about this now?

In general, if you care about precise types you really should be using --strictNullChecks (where we type this correctly).

[RyanCavanaugh]

I don't recall that discussion. We could check RWC for impact - seems like we'd reveal a ton of subtle bugs

[RyanCavanaugh]

RWC results were nothing but false positives. Doesn't seem worth breaking at this point.