Make 'pop' the first method that uses 'T' in 'Array'

[Daniel Rosenwasser (DanielRosenwasser)]

Chatted with Mohamed Hegazy (@mhegazy) about this, but I'd like to get your thoughts on this Anders Hejlsberg (@ahejlsberg) since you sent out #436.

When elaborating errors on an Array<Foo> to an Array<Bar>, diving into a structural comparison is often dizzying for the user since the first elaboration occurs on push.

Since push uses T contravariantly, we'll end up flipping the direction which is much more difficult for users to parse. Furthermore, in cases where the covariant check would provide better error messages and error message spans (such as excess property checking spans, and picking better branches in unions), we would end up doing significantly worse.

As an example from Cliff Koh (@cliffkoh) and David Zearing (@dzearing), the following

typeIStyle=IStyleBase|IStyleArray;interfaceIStyleArrayextendsArray<IStyle>{}interfaceIStyleBase{foo: string;}constblah: IStyle=[[[{foo: 'asdf',jj: 1// intentional error}]]];

Provides an error on blah:

const blah: IStyle = [
~~~~
[ts] Type '{ foo: string; jj: number; }[][][]' is not assignable to type 'IStyle'.
Type '{ foo: string; jj: number; }[][][]' is not assignable to type 'IStyleArray'.
Types of property 'push' are incompatible.
Type '(...items: { foo: string; jj: number; }[][][]) => number' is not assignable to type '(...items: IStyle[]) => number'.
Types of parameters 'items' and 'items' are incompatible.
Type 'IStyle' is not assignable to type '{ foo: string; jj: number; }[][]'.
Type 'IStyleBase' is not assignable to type '{ foo: string; jj: number; }[][]'.
Property 'length' is missing in type 'IStyleBase'.

Whereas with this change, we provide an error on jj (the excess property):

 jj: 1 // intentional error
~~
[ts] Type '{ foo: string; jj: number; }[][][]' is not assignable to type 'IStyle'.
Type '{ foo: string; jj: number; }[][][]' is not assignable to type 'IStyleArray'.
Types of property 'pop' are incompatible.
Type '() => { foo: string; jj: number; }[][]' is not assignable to type '() => IStyle'.
Type '{ foo: string; jj: number; }[][]' is not assignable to type 'IStyle'.
Type '{ foo: string; jj: number; }[][]' is not assignable to type 'IStyleArray'.
Types of property 'pop' are incompatible.
Type '() => { foo: string; jj: number; }[]' is not assignable to type '() => IStyle'.
Type '{ foo: string; jj: number; }[]' is not assignable to type 'IStyle'.
Type '{ foo: string; jj: number; }[]' is not assignable to type 'IStyleArray'.
Types of property 'pop' are incompatible.
Type '() => { foo: string; jj: number; }' is not assignable to type '() => IStyle'.
Type '{ foo: string; jj: number; }' is not assignable to type 'IStyle'.
Object literal may only specify known properties, and 'jj' does not exist in type 'IStyle'.

This is horrifyingly longer, but it gives the right span and gets to the root cause at the end.

[Daniel Rosenwasser (DanielRosenwasser)]

Really the RWC baseline changes are minimal, and aren't impacted at all. I'm just seeing reports of missing pop instead of push:

 Argument of type 'Uint8ClampedArray' is not assignable to parameter of type 'any[]'.
+ Property 'pop' is missing in type 'Uint8ClampedArray'.- Property 'push' is missing in type 'Uint8ClampedArray'.

[Mohsen Azimi (mohsen1)]

Can you improve those errors for Arrays even further. I got confused with code like this:

interfaceValues{textValues: {values: string[]}}constitems=[{name: 'a'},{name: 'b'}]varvalues: Values={textValues: items.map(i=>i.name)}

Type 'string[]' is not assignable to type '{ values: string[]; }'.
Types of property 'values' are incompatible.
Type '() => IterableIterator<string>' is not assignable to type 'string[]'.
Property 'pop' is missing in type '() => IterableIterator<string>'