Type predicate results stored in variables don't narrow guarded types

[bgmort]

TypeScript Version: 2.8.0-dev.20180222

Search Terms:
assignable type guards
assignable type guard return value
assign type guard to variable

Code

interface A {
    type: 'A';
    aProperty: string;
}

interface B {
    type: 'B';
    bProperty: number;
}

type AorB = A | B;

function isA(obj: AorB): obj is A {
    return obj.type === 'A';
}

const o = { type: 'A', aProperty: 'value' } as any as AorB;

// this should be an error, and it is
console.log(o.aProperty);

// this should work, and it does
if (isA(o)) {
    console.log(o.aProperty);
}

// this example discards the type guard information, but it 
// ought to set oIsA to type (o is A), and apply the type
// guard inside the if statement. 
const oIsA = isA(o);
if (oIsA) {
    console.log(o.aProperty);
}

Expected behavior:
oIsA should have the type returned by the type guard function. if (oIsA) ... should narrow the type of o within the if block.

Actual behavior:
oIsA is a regular boolean value. The type guard function can only be used directly within a conditional block.

Playground Link:
https://goo.gl/6Pk5vr

Related Issues:

[DanielRosenwasser]

I can't find the other duplicates, but we only check syntactically whether isA is a user-defined type guard at the if check (and other syntactic locations). The type predicate return type is really just a boolean in any other context. So oIsA doesn't track any information about the type of o itself.

We've considered this problem, but it seems fairly complex and expensive to solve in a general way.

[bgmort]

@DanielRosenwasser I found the duplicates neither of us could find the first time around. This one seems to be the most similar: #14891

[mhegazy]

I think #14891 is a different issue..

the is a duplicate of #15482 #14630, #10976, #12263, #10065, and #10846

[typescript-bot]

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