Incorrect handling of unions of arrays.

[domoritz]

TypeScript Version: 2.1

Code

This code works although the type of the forEach callback is not suffuient.

const a: boolean[] | number[] = [1,1,1];
a.forEach((b: number) => console.log);

But this fails (which is expected)

const a: boolean[] | number[] = [1,1,1];
a.forEach((b: boolean) => console.log);

I'd expect only this code to be valid

const a: boolean[] | number[] = [1,1,1];
a.forEach((b: boolean | number) => console.log);

I understand that boolean[] | number[] is a subtype of (boolean | number)[] and would expect typescript to be stricter if I provide the former.

[ahejlsberg]

Your first example compiles because control flow analysis has determined the type of a to be number[] at the call to forEach because the preceding assignment assigned an array literal of type number[].

[domoritz]

This is the smallest example I could create to recreate an issue.

In

channelEncoding.forEach(function(fieldDef: FieldDef | OrderChannelDef) {
  fields.push(field(fieldDef));
});

I know that channelEncoding is a FieldDef[] | OrderChannelDef[].

But I get the following error

[ts] Cannot invoke an expression whose type lacks a call signature. Type '((callbackfn: (value: FieldDef, index: number, array: FieldDef[]) => void, thisArg?: any) => void...' has no compatible call signatures.

Yes, the error is slightly different but I thought that was because ts handles complex types differently from simple types.

I thought this error is caused by the behavior of the flow analysis (which is being smarter than my explicit type annotations).

[ahejlsberg]

This error is reported because forEach has a different signature in each of the two arrays, so there is not a common signature we can use in the call expression. You can help by providing a type assertion:

(channelEncoding as (FieldDef | OrderChannelDef)[]).forEach(...)

Alternatively, you could change channelEncoding to have type (FieldDef | OrderChannelDef)[]. It is slightly less strict (because it allows mixed arrays), but probably easier to work with.

[ahejlsberg]

For more discussion see #10620.

[domoritz]

Thanks for the explanation! The behavior I posted above is still a bit odd (flow analysis overrides my explicit annotation) but I don't have a problem with it.