HOC returned component props can not differ from HOC generic props

TypeScript Version: 3.3.0-dev.20181208

Search Terms:

• HOC
• react
• higher order component

Code

import * as React from 'react';

export interface HOCProps {
  foo: number;
}

/** Remove props, that have been prefilled by the HOC */
type WithoutPrefilled<T extends HOCProps> = Pick<T, Exclude<keyof T, 'foo'>>;

function withFoo<P extends HOCProps>(WrappedComponent: React.ComponentType<P>) {
  return class SomeHOC extends React.Component<WithoutPrefilled<P>> {
    public render(): JSX.Element {
      return <WrappedComponent {...this.props} foo={0} />;
    }
  };
}

Expected behavior: No error, like with every version below 3.2.0.

Actual behavior: Throws an error highlighting the WrappedComponent in the render method.

[ts]
Type 'Readonly<{ children?: ReactNode; }> & Readonly<Pick<P, Exclude<keyof P, "foo">>> & { foo: number; }' is not assignable to type 'IntrinsicAttributes & P & { children?: ReactNode; }'.
  Type 'Readonly<{ children?: ReactNode; }> & Readonly<Pick<P, Exclude<keyof P, "foo">>> & { foo: number; }' is not assignable to type 'P'. [2322]

Additional Information

This is pretty much the same sample example used in https://github.com/Microsoft/TypeScript/issues/28720, but with the difference, that the props of the returned component differ from the generic.

Basically the HOC prefills the foo property for the WrappedComponent. Since the spreaded props are overriden by foo, I don’t want foo to be a valid property for the HOC. This does not seem to be possible anymore.

Playground Link:

Related Issues: https://github.com/Microsoft/TypeScript/issues/28720

[ChristianIvicevic]

Starting with 3.2 the behaviour of the spread operator for generics has changed. Apparently the type of props gets erased as a negative side effect, but you can work around that by casting it back to P using {...props as P} when spreading back into the wrapped component.

[weswigham]

@ahejlsberg The change is that we no longer erase generics in JSX (so we actually check these calls now), and roughly that Pick<P, Exclude<keyof P, "foo">> & { foo: P["foo"] } doesn’t recombine to (or get recognized as assignable to) P. It’s an unfortunate interaction with generic rest/spread, and the error we output is bad, too.