[addon-controls] Decorators aren't able to add controls

[andyrichardson]

Describe the bug
Adding argTypes on a story doesn't result in any options being presented in controls panel.

To Reproduce

Consider the following decorator which provides a value via context which can be changed via a control.

const userDecorator: DecoratorFn = (Story, context) => (
  <UserContext.Provider value={context.args.user}>
    <Story 
      {...context} 
      argTypes={{ ...context.argTypes, user: { control: 'text' } }} // argType is added to story
    />
  </UserContext.Provider>
)

Expected behavior
The user control, added by the decorator, is presented in the controls panel.

Actual behavior
Any changes to the argTypes attribute in context or context.parameters makes no difference.

System

Environment Info:

  System:
    OS: macOS 10.15.6
    CPU: (12) x64 Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
  Binaries:
    Node: 12.18.0 - ~/.nvm/versions/node/v12.18.0/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.4 - ~/.nvm/versions/node/v12.18.0/bin/npm
  Browsers:
    Chrome: 86.0.4240.80
    Edge: 86.0.622.43
    Firefox: 80.0.1
    Safari: 14.0
  npmPackages:
    @storybook/addon-actions: ^6.0.26 => 6.0.26
    @storybook/addon-controls: ^6.0.26 => 6.0.26
    @storybook/addon-essentials: ^6.0.26 => 6.0.26
    @storybook/addon-knobs: ^6.0.26 => 6.0.26
    @storybook/addon-links: ^6.0.26 => 6.0.26
    @storybook/addon-storyshots: ^6.0.26 => 6.0.26
    @storybook/addon-storyshots-puppeteer: ^6.0.26 => 6.0.26
    @storybook/addons: ^6.0.26 => 6.0.26
    @storybook/preset-create-react-app: ^3.1.4 => 3.1.4
    @storybook/react: ^6.0.26 => 6.0.26

Additional context
This bug only occurs when setting argTypes in a decorator.

The following works fine:

export const MyStory = () => <SomeComponent />
MyStory.argTypes = { user: { control: 'text' } };

[shilman]

argTypes are defined statically, so the only way to extend them is to annotate a story or the default export in a story file. they can't be extended dynamically with decorators.

[andyrichardson]

@shilman is that something that is open to change? My understanding was that an addon only parses a story once all the decorators have been applied.

Does this mean users are expected to sue both knobs and argTypes?

[andyrichardson]

Closing as this is a dupe of #11984