QuickPickOptions unusable typings small fix

[amadare42]

When calling vscode.window.showQuickPick with custom type items, onDidSelectItem callback cannot be correctly typed:

interface MyPickItem extends vscode.QuickPickItem { }
var items:MyPickItem[];
vscode.window.showQuickPick(items, { 
    placeHolder: "(placeholder)", 
    onDidSelectItem: (item:MyPickItem) => {
        /* Wont copile, error:
            Argument of type '{ placeHolder: string; onDidSelectItem: (item: MyPickItem) => void; }' is not assignable to parameter of type 'QuickPickOptions'.
            Types of property 'onDidSelectItem' are incompatible.
                Type '(item: MyPickItem) => void' is not assignable to type '<T extends QuickPickItem>(item: string | T) => any'.
                Types of parameters 'item' and 'item' are incompatible.
                    Type 'string | T' is not assignable to type 'MyPickItem'.
                    Type 'string' is not assignable to type 'MyPickItem'.
        */
    }
});

That because of way which typescript handles generic constraints ( microsoft/TypeScript#3410 (comment) ). So easy fix to make it usable is to just change constraint to base QuickPickItem interface.

[mention-bot]

@amadare42, thanks for your PR! By analyzing the history of the files in this pull request, we identified @egamma and @jrieken to be potential reviewers.

[msftclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
Microsoft Pull Request Bot

[jrieken]

@mhegazy Can you shed some light on this? This is the signature in question: onDidSelectItem?<T extends QuickPickItem>(item: T | string): any; Is this just because TypeScript doesn't support the usage of generic types in function and its params anymore. It sounds like a shortcut has been taken and the consequence is that we have to change (break?) our API

[jrieken]

Merging for being compile errors free, tho I still don't understand this. @mjbvz Can you bring this up in the next TS sync?

[mjbvz]

Thanks. I've opened microsoft/TypeScript#17432 to track this issue