TypeScript: can't infer prop names and types when contains any type prop

[wonderful-panda]

Version

2.5.2

Reproduction link

https://github.com/wonderful-panda/vue-sandbox

Steps to reproduce

Compile this code by tsc

import Vue, { VNode } from "vue";

const NgComponent = Vue.extend({
    name: "NgComponent",
    props: {
        foo: { type: String },
        bar: {}    // any types are accepted
    },
    render(h): VNode {
        return h("div", this.foo);
    }
});

or clone from repro link and npm install && npm run build

What is expected?

Successfully compiled.

And this.foo is treated as string, this.bar is treated as any.

What is actually happening?

Failed to compile.

src/index.ts(10,30): error TS2339: Property 'foo' does not exist on type 'CombinedVueInstance<Vue, {}, {}, {}, Readonly<{}>>'.

---

Workaround:

import Vue, { VNode } from "vue";

Vue.extend({
    name: "Workaround",
    props: {
        foo: { type: String },
        bar: {} as (() => any)   // any types are accepted
    },
    render(h): VNode {
        return h("div", this.foo);
    }
});

[ktsn]

Indeed it does not work if it has any typed props definition. I'm not sure about the reason, though.

@HerringtonDarkholme @DanielRosenwasser
I would appreciate if you can look into it.

[DanielRosenwasser]

I'm currently not by a computer but I'll guess a bit. Right now it's probably an assignability error for props in general, but we can do better there in future versions of TypeScript. I'm currently on a branch where this works better.

The thing is that even on this branch, the best inference I can get is {}.

[HerringtonDarkholme]

With #6850 , you can do this.

import Vue, { VNode } from "vue";

Vue.extend({
    name: "Workaround",
    props: {
        foo: { type: String },
        bar: {} as Prop<any>
    },
    render(h): VNode {
        return h("div", this.foo);
    }
});

I think inferring any from empty prop type is too lenient. A lot of options will have empty field, which results in empty type parameter. If we make inference default to any, there will be too many errors hidden by unintended any.

[yyx990803]

Closing this one - please keep an eye on #6856 (to be merged for 2.6)