Type of prop with a default value is incorrect

[sylvaingi]

When upgrading to v1.0.0-rc.4, I'm getting a lot of type errors on components using props with a default value.
It looks like #655 introduced this regression.

Using the following component, the props argument of setup() is typed differently between rc.3 and rc.4:

export default defineComponent({
  props: {
    id: { type: String, default: 'def' },
    count: { type: Number, required: true },
  },
  setup(props) {},
});

rc.3:

props: {
    id: string;
    count: number;
} & {}

rc.4:

props: {
    count: number;
} & {
    id?: string | undefined;
}

rc.4 is incorrect here as the id prop will never be undefined at runtime, as it receives the default value if one wasn't provided.