Invariant Violation: Hooks can only be called inside the body of a function component.

Error description

Invariant Violation: Hooks can only be called inside the body of a function component.

      2 | 
      3 | export function useFormField(initialValue) {
    > 4 |   const [value, setValue] = useState(initialValue)
        |                             ^
      5 | 
      6 |   function onChange(e) {
      7 |     setValue(e.target.value)

      at invariant (node_modules/react/cjs/react.development.js:125:15)
      at resolveDispatcher (node_modules/react/cjs/react.development.js:1424:28)
      at useState (node_modules/react/cjs/react.development.js:1447:20)
      at useFormField (components/hooks/hooks.js:4:29)
      at Object.<anonymous> (components/hooks/__tests__/hooks-spec.js:8:20)

Dep:
```
"next": "latest",
"react": "16.7.0-alpha.2",
"react-dom": "16.7.0-alpha.2",
"react-testing-library": "^5.4.2"
```

hooks.js

import { useState } from 'react'

export function useFormField(initialValue) {
  const [value, setValue] = useState(initialValue)

  function onChange(e) {
    setValue(e.target.value)
  }

  return {
    value,
    onChange,
  }
}

my-custom-hook-spec.js

import { render, fireEvent } from 'react-testing-library'
import { useFormField } from '../hooks'

describe('Hooks', () => {
  describe('useFormField with input', () => {
    it.skip('should set value and listen to type event', () => {
      const { container } = render(
        <input {...useFormField('')} />
      )
      const input = container.firstChild

      fireEvent.change(input, {target: {value: 'abc'}})

      expect(input.value).toBe('abc')
    })
  })
})