Tests should reject unqualified use of undefined interfaces

[BurtHarris]

The test tool seems to allow definitions to use undefined and unqualified interfaces like IterableIterater<T>, Symbol, and even ReadableStream.

Should it?! This seems to lead to issues like DefinitelyTyped/DefinitelyTyped#10919.

[mhegazy]

totally misread it. before we do that we need to make sure all libraries can abide by it. not sure this would be the case, at least for some of the polyfills like core-js that really need to define Symbol.

[BurtHarris]

As I remember the problems revolved around dependency on these symbols, not definition of them. That said, how polyfills should work for TypeScript is still very unclear to me, but I'm not saying that the .d.ts for a polyfill shouldn't be able to define Symbol, rather that unqualified (and undefined) references to new types like Symbol (and the like) seem like to leave the question of how/when to introduce the polyfill definition unanswered.

Perhaps a .d.ts file should declare what minimum version of ECMAScript (or polyfills) it requires.

[mhegazy]

As I remember the problems revolved around dependency on these symbols, not definition of them.

Can you elaborate?

[BurtHarris]

Sure @mhegazy, I'll try again. Lets take issue DefinitelyTyped/DefinitelyTyped#10919 for example. It appears that a change to the node package's definition that introduced use of IterableIterator. This commit passed the integration tests, presumably because they were processing it using ECMAScript 2015 mode. But this change causes surprising errors for typescript users still emitting ES5 code because IterableIterator is not declared anywhere they can see.

I'm suggesting that perhaps something should be done to prevent the hard-to-interpret error that results when an existing definitions file is upgraded to take a new dependency on ES2015. I.e. maybe there should be some qualifier in the package that explicitly says if it requires ES2015, (or a polyfill.)

But no matter how that's resolved, I wasn't proposing generating errors for a polyfill(s) that define IterableIterator. On the contrary, I'd like to see some sort of conditional import feature that includes the polyfill only if we are not emitting ES2015. Does that make sense? Please feel free to retitle the issue if that's called for...