No string literal type for optional property in JSX

TypeScript Version: 2.7.0-dev.20180104

Code

import * as React from "react";

interface Props {
    x?: "a" | "b";
}
class MyComponent<P extends Props = Props> extends React.Component<P> {}
const m = <MyComponent x="a"/>

Expected behavior:

No error.

Actual behavior:

src/a.tsx(7,24): error TS2322: Type '{ x: string; }' is not assignable to type 'IntrinsicAttributes & IntrinsicClassAttributes<MyComponent<Props>> & Readonly<{ children?: ReactN...'.
  Type '{ x: string; }' is not assignable to type 'Readonly<Props>'.
    Types of property 'x' are incompatible.
      Type 'string' is not assignable to type '"a" | "b" | undefined'.

This was working in 2.7.0-dev.20180103.
There is no error if x is made non-optional.
But discovered in react-leaflet on DefinitelyTyped. There are also errors in react-router that look similar.