Rest route index page crash on compile

[IlyaSemenov]

Extracted from #706 as it's an unrelated issue.

• /routes/[...categories]/[item].html.svelte - this one works
• neither /routes/[...categories].svelte or /routes/[...categories]/index.svelte works:

❯ yarn dev
yarn run v1.16.0
warning package.json: No license field
$ sapper dev
✗ client
src/routes/[...categories]/index.svelte
Module build failed (from ./node_modules/svelte-loader/index.js):
Error: TypeError: TypeError: Cannot read property 'toUpperCase' of undefined
✗ server
src/routes/[...categories]/index.svelte
Module build failed (from ./node_modules/svelte-loader/index.js):
Error: TypeError: TypeError: Cannot read property 'toUpperCase' of undefined
✔ service worker (168ms)
webpack:///./src/routes/%5B...categories%5D/index.svelte?:1
throw new Error("Module build failed (from ./node_modules/svelte-loader/index.js):\nError: TypeError: TypeError: Cannot read property 'toUpperCase' of undefined\n    at preprocess.then.catch.err (/Users/semenov/tmp/my-app/node_modules/svelte-loader/index.js:180:12)");
^

Error: Module build failed (from ./node_modules/svelte-loader/index.js):
Error: TypeError: TypeError: Cannot read property 'toUpperCase' of undefined
    at preprocess.then.catch.err (/Users/semenov/tmp/my-app/node_modules/svelte-loader/index.js:180:12)
    at eval (webpack:///./src/routes/%5B...categories%5D/index.svelte?:1:7)
    at Object../src/routes/[...categories]/index.svelte (/Users/semenov/tmp/my-app/__sapper__/dev/server/server.js:193:1)
    at __webpack_require__ (/Users/semenov/tmp/my-app/__sapper__/dev/server/server.js:21:30)
    at eval (webpack:///./src/node_modules/@sapper/internal/manifest-server.mjs?:12:89)
    at Module../src/node_modules/@sapper/internal/manifest-server.mjs (/Users/semenov/tmp/my-app/__sapper__/dev/server/server.js:158:1)
    at __webpack_require__ (/Users/semenov/tmp/my-app/__sapper__/dev/server/server.js:21:30)
    at eval (webpack:///./src/node_modules/@sapper/server.mjs?:5:83)
    at Module../src/node_modules/@sapper/server.mjs (/Users/semenov/tmp/my-app/__sapper__/dev/server/server.js:182:1)
    at __webpack_require__ (/Users/semenov/tmp/my-app/__sapper__/dev/server/server.js:21:30)
    at eval (webpack:///./src/server.js?:8:72)
> Server crashed

Tested on a brand new install of npx degit sveltejs/sapper-template#webpack my-app with only one file added at src/routes/[...categories]/index.svelte:

<h1>cat index</h1>

---

I traced this to be caused by Svelte's code:

    function get_name(filename) {
        if (!filename)
            return null;
        const parts = filename.split(/[\/\\]/);
        if (parts.length > 1 && /^index\.\w+/.test(parts[parts.length - 1])) {
            parts.pop();
        }
        const base = parts.pop()
            .replace(/\..+/, "") // <------ here [...categories] becomes [
            .replace(/[^a-zA-Z_$0-9]+/g, '_') // <------ here it becomes _
            .replace(/^_/, '') // <------ here it becomes empty
            .replace(/_$/, '')
            .replace(/^(\d)/, '_$1');
        return base[0].toUpperCase() + base.slice(1); // here it crashes
    }

[jumpinggoofballs]

I get the same thing on the rollup template.

[Conduitry]

As noted in the linked issue, this is a Svelte bug. Closing this one.