Svelte component documentation not generated when using inline arrow functions (() => () => {})

[jogarijo]

Describe the bug
In a Svelte project, Storybook fails to generate a component documentation when it contains nested inline arrow functions (i.e. () => () => {}).

To Reproduce
Steps to reproduce the behavior:

1. Create a simple Svelte project with Storybook. You can bootstrap one using npx degit chromaui/intro-storybook-svelte-template taskbox for example;
2. Install dependencies;
3. Run storybook (npm run storybook or yarn storybook);
4. In any Svelte file, add an arrow function like in the "Code snippets" section below in one of the components;
5. You should see an error in the console (see "Additional context"). Additionally, the component documentation will be empty.

Expected behavior
There should be no error

Code snippets

<script>
const foo = () => () => {} // fails
// works
const bar = () => {
  return () => {}
}}
</script>

System

  Binaries:
    Node: 14.8.0
    Yarn: 1.22.10
    npm: 6.14.7
  npmPackages:
    @storybook/addon-docs: ^6.1.21 => 6.1.21 
    @storybook/addon-essentials: ^6.1.21 => 6.1.21 
    @storybook/addon-links: ^6.1.21 => 6.1.21 
    @storybook/svelte: ^6.1.21 => 6.1.21 

Additional context
The following error is displayed by Webpack during the build:

TypeError: nodes.forEach is not a function
    at Parser.parseBodyRecursively (<path>/node_modules/sveltedoc-parser/lib/v3/parser.js:251:15)
    at <path>/node_modules/sveltedoc-parser/lib/v3/parser.js:331:38
    at Array.forEach (<anonymous>)
    at <path>/node_modules/sveltedoc-parser/lib/v3/parser.js:311:27
    at Array.forEach (<anonymous>)
    at Parser.parseBodyRecursively (<path>/node_modules/sveltedoc-parser/lib/v3/parser.js:251:15)
    at Parser.parseScriptBlock (<path>/node_modules/sveltedoc-parser/lib/v3/parser.js:571:14)
    at <path>/node_modules/sveltedoc-parser/lib/v3/parser.js:72:22
    at Array.forEach (<anonymous>)
    at Parser.__walk (<path>/node_modules/sveltedoc-parser/lib/v3/parser.js:71:36)
    at <path>/node_modules/sveltedoc-parser/lib/v3/parser.js:56:22

This error occurs in sveltedoc-parser in versions <4.1.0. Storybook uses version 3.0.5, and forcing the dependency to ^4.1.0 seem to solve the issue.

[shilman]

Egads!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.2.0-beta.11 containing PR #14164 that references this issue. Upgrade today to the @next NPM tag to try it out!

npx sb upgrade --prerelease

Closing this issue. Please re-open if you think there's still more to do.