Recent --strictNullChecks issue w/ && || type result check

[GoToLoop]

About 1 weak ago, this sample below would transpile flawlessly under --strictNullChecks:

let sinOrCos = Math.random() < .5;

let choice = sinOrCos && Math.sin || Math.cos;
//let choice = sinOrCos? Math.sin : Math.cos;

alert(choice(Math.PI));

But to my surprise today, under [email protected], only the statement using the ternary conditional operator works now!

Somehow TS thinks that choice could be boolean as well when using && + || expression. =(
Is that intentional from now on? I miss when it was smarter just 1 week ago. :-(

tsc --pretty --newLine LF --removeComments --experimentalDecorators --strictNullChecks -d -t ES6 sinCos

[GoToLoop]

1 more example:

function sq(n?: number): number {
  const r = n !== undefined && n*n || 0;
  return r;
}

alert(sq(3));

sq.ts(3,9): error TS2322: Type 'number | boolean' is not assignable to type 'number'.
Type 'boolean' is not assignable to type 'number'.

[kitsonk]

Like due to changes in #8949 which resolved #8892.

[ahejlsberg]

I see what's going on here. The issue is that we give the sub-expression sinOrCos && Math.sin the type ((x: number) => number) | boolean. Since the result could be the boolean value false, this is technically not wrong. But it is a little bit too permissive. The type really should be ((x: number) => number) | false, meaning that the only the boolean value false is possible. We should likewise return T | 0 and T | "" for && operations where the first operand is of type number or string respectively.

Then we should furthermore say that in an x || y operation, we remove the definitely-falsy types (undefined, null, void, false, 0, and "") from the type of x in the resulting union type.

Now, the only problem with this solution is that we don't (yet) support boolean literal types or numeric literal types. This means we need to do one of the following:

• Add support for boolean and numeric literal types now.
• Back out Improve typing of && operator with --strictNullChecks #8949 and add support for boolean and numeric literal types later.

Adding @mhegazy and @RyanCavanaugh.

[zpdDG4gta8XKpMCd]

Add support for boolean and numeric literal types now.

👍

[zpdDG4gta8XKpMCd]

and numeric ranges:

type Positive = number > 0;

[GoToLoop]

Numeric ranges? Are those even possible? That'd be über cool though.
I always wanted to use boolean literals for overloaded functions just like strings do.
I wonder whether those proposed numeric literals would include enums as well?
But of course the most important thing now is to have the && + || archaic idiom back under --strictNullChecks! :P