[Bug]: [addon-docs] MDX stories not working at all

[ayZagen]

Describe the bug

I tried the code example in code/addons/docs/vue3/README.md#mdx but it does not work at all.

I tried with both 8.4.7 and 8.5.0-alpha.20 both versions fails. Also without creating mdx file which does not throw the error Docs tab is not shown at all.

Following error is thrown:

Unable to index ./src/stories/Test.stories.mdx:
WARN   Error: Invariant failed: No matching indexer found for /home/projects/llqiwrnnox.github/src/stories/Test.stories.mdx
WARN     at invariant (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34558:11)
WARN     at Si.extractStories (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34758:5)
WARN     at eval (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34703:53)
WARN     at eval (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34679:30)
WARN     at eval (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34676:26)
WARN     at Si.updateExtracted (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34668:23)
WARN     at Si.ensureExtracted (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34702:16)
WARN     at Si.initialize (/home/projects/llqiwrnnox.github/node_modules/@storybook/core/dist/core-server/index.cjs:34663:16)

Reproduction link

https://stackblitz.com/edit/github-tyj2nvxh?file=src%2Fstories%2FTest.stories.mdx

Reproduction steps

• add addon-docs addon
• create an mdx file like in example

System

Storybook Environment Info:

  System:
    OS: Linux 5.15 Ubuntu 24.04.1 LTS 24.04.1 LTS (Noble Numbat)
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
    Shell: 5.9 - /usr/bin/zsh
  Binaries:
    Node: 22.12.0 - ~/.local/share/mise/installs/node/22.12.0/bin/node
    npm: 10.9.0 - ~/.local/share/mise/installs/node/22.12.0/bin/npm
    pnpm: 9.15.0 - ~/.local/share/mise/installs/node/22.12.0/bin/pnpm <----- active
  npmPackages:
    @storybook/addon-docs: ^8.4.7 => 8.4.7 
    @storybook/addon-essentials: ^8.4.7 => 8.4.7 
    @storybook/addon-interactions: ^8.4.7 => 8.4.7 
    @storybook/addon-onboarding: ^8.4.7 => 8.4.7 
    @storybook/blocks: ^8.4.7 => 8.4.7 
    @storybook/test: ^8.4.7 => 8.4.7 
    @storybook/vue3: ^8.4.7 => 8.4.7 
    @storybook/vue3-vite: ^8.4.7 => 8.4.7 
    storybook: ^8.4.7 => 8.4.7

Additional context

No response