[Bug]: Type errors in stories where component has discriminated union in its prop type definition, and custom properties are used

[selrond]

Describe the bug

It seems to be impossible to properly type a story, when a component

1. has discriminated union in its props type definition, and
2. its story contains custom storybook-only property

Reproduction link

https://stackblitz.com/edit/storybook-type-errors?file=src%2Fstories%2FComponent.tsx,src%2Fstories%2FComponent.stories.tsx

Reproduction steps

No response

System

Storybook Environment Info:

  System:
    OS: macOS 15.2
    CPU: (14) arm64 Apple M4 Pro
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.11.0 - ~/.local/state/fnm_multishells/52967_1737709857083/bin/node
    npm: 10.9.0 - ~/.local/state/fnm_multishells/52967_1737709857083/bin/npm
    pnpm: 9.14.2 - ~/.local/state/fnm_multishells/52967_1737709857083/bin/pnpm <----- active
  Browsers:
    Chrome: 132.0.6834.110
    Safari: 18.2
  npmPackages:
    @storybook/addon-docs: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/addon-essentials: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/addon-interactions: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/addon-themes: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/blocks: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/nextjs: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/react: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    @storybook/test: ^8.6.0-alpha.2 => 8.6.0-alpha.2
    eslint-plugin-storybook: ^0.11.2 => 0.11.2
    storybook: ^8.6.0-alpha.2 => 8.6.0-alpha.2

Additional context

Originally found on v8.5.0 of storybook packages, tried to upgrade to latest versions to see if it’s fixed.

[selrond]

Maybe related to this #25492

[valentinpalkovic]

Hi @selrond

Thank you for reporting the issue.

Can you please try using react-docgen-typescript instead of react-docgen for getting the proper type extracted from your component? Resource: https://storybook.js.org/docs/configure/integration/typescript#the-types-are-not-being-generated-for-my-component