Confusing error message: Operator '===' cannot be applied to types '1' and '0'.

[mjbvz]

TypeScript Version: 3.0.0-dev.20180623

Search Terms:

Code

function foo(x: [number]) {
    return x.length === 0;
}

Bug:
The error message is: Operator '===' cannot be applied to types '1' and '0'.

I found this error very confusing. It took me a while to understand that the error meant that the type on the left hand side was 1.

In this case, I think we should instead report that the condition always evaluates to false.

Related Issues:

• true == false - cannot be applied to types 'false' and 'true'. #11178

/cc @chrisdias

[RyanCavanaugh]

We should have a sub-elaboration when both operands are literal types (or unions of literal types) that says something like

The types of the values indicate this comparison is always 'false'

and is

The types of the operands appear to be unrelated

in the general case

[RyanCavanaugh]

@DanielRosenwasser let's workshop it for a few hours 🎈

[jeffreymorlan]

The new error is misleading. You can't compare unrelated interfaces, but it's still possible and reasonable that one object could implement both of them:

const a = { x: 0, y: 0 };
const b: { x: number } = a;
const c: { y: number } = a;
console.log(b == c);

but now it claims that "The types of these values indicate that this condition will always be 'false'."

[RyanCavanaugh]

@DanielRosenwasser - @jeffreymorlan makes a good point here, I was expecting that we'd implement the "when both operands are literal types (or unions of literal types)" constraint for this message.