This allows you to mix the types for multiple jsx factories in one compilation in a similar way to how you can now mix factory functions. What this change does is look for an exported JSX namespace under the "react namespace" of the factory function. What this means is that if the factory function is React.createElement, we'll look for React.JSX. If the factory function is p, we'll look for p.JSX. If a local JSX namespace cannot be found, we still fallback to the global one, if present. This does not change how we discover any JSX types beyond looking for them in an additional location.

An an example, react's core exports can now be defined like so:

export function createElement<P>(/*complex types*/): Component<P>;
export namespace createElement {
    export namespace JSX {
        interface Element extends ReactElement<any> {}
        interface ClassElement extends Component<any> {}
        // ... and so on
    }
}

and you get everything you need on the consuming end with your standard

import * as React from "react";
export const MyComponent = (p: {x?: number, y?: number}) => <h1>/*...*/</h1>;

With this change, a JSX package no longer needs to pollute the global scope with types that can conflict with other packages. 😄

Fixes #18131
IMHO, this elegantly solves the aliasing problem posed in the original issue (we're just doing the lookup inside the factory function's namespace, rather than trying to trace back into the file it came from and looking beside it)