CC @donataswix

It appears that using a React component written as a class in a .js file causes some issues.

Start a project with the following files:

tsconfig.json

{
  "compilerOptions": {
    "module": "es2015",
    "target": "esnext",
    "jsx": "react-native",
    "strictNullChecks": true,
    "allowSyntheticDefaultImports": true,
    "allowJs": true,
    "lib": [
      "es5",
      "es6",
      "es7",
      "es2017",
      "dom"
    ],
    "outDir": "dist"
  },
  "include": [
    "**/*.ts",
    "**/*.tsx"
  ],
  "exclude": [
    "node_modules"
  ]
}

index.tsx

import React from 'react';
import Hello from './hello';

class A extends React.Component<void,void> {
  render() {
    return (
      <div>
        <Hello/>
      </div>
    );
  }
}

hello.js

import React from 'react';

export default class Hello extends React.Component {
  render() {
    return <div>Hello</div>;
  }
}

Expected: No problems
Actual: Error on <Hello />: JSX element type 'Hello' does not have any construct or call signatures.

Note that if you add type arguments to Component in hello.js, this will actually fix the problem (while creating new ones).

Potentially related is #13609.