Should not widen in type argument inference

[JsonFreeman]

Compile the following using noImplicitAny:

function foo<T>(x: T): T { return x }

interface I {
    x: string;
}

var i: I = foo({ x: null }); // Error
var s: string = f(null); // No error

Neither of these should be errors. According to the spec, both of these should widen, but in #531 @DanielRosenwasser and I explain why these should not widen.

The noImplicitAny error resulting from widening is a breaking change from 1.0, and it broke a customer.

[ahejlsberg]

We have to widen because foo can return an arbitrary (circular) type in which the null or undefined gets buried. We would require a new algorithm to traverse such a type to find and widen the null and undefined types. Right now we know that the types we widen are never circular.

[DanielRosenwasser]

Could you give an example of such a type just to document on this issue?

[JsonFreeman]

I think the following code would break if we do this:

declare function makeArray<T>(x: T): T[];
makeArray(null)[0] = 0; // Error that number is not assignable to null

I am not sure if there are useful examples where the user would blocked by this. But the point is, it leads to null and undefined leaking out in a lot more places. The cost of doing this would involve taking inventory of all the places where this could happen and widening in all those places.

[JsonFreeman]

Then again, the following code is already an error because of a null leak:

[null][0] = 0;

So this may not really have to do with type argument inference at the core.

[osdm]

Hi @JsonFreeman ! You say "According to the spec...". Could you point me, where does the spec says about widening in the type inference? I searched through 1.3, 1.4 and 1.5 versions of the spec and found nothing about widening in the type inference. Maybe it's just me being dumb...

[JsonFreeman]

Hmm, now I can't find it anymore. I remember it being in the second bullet point in section 4.12.2 (Type argument inference).

@ahejlsberg, was it intentional that widening at the end of type argument inference was removed from the spec?

[osdm]

@JsonFreeman Ok, I was wrong about 1.3, sorry. Widening is really there. But it disappeared in 1.4.

[JsonFreeman]

Ok, I see it was removed in 7a2701b. I think this was just an oversight, as the definition of C moved inside the bullet. The change we were trying to make was just to add the capability for type argument inference to error. This was at the time we introduced union types.