Ancestor directory names containing a leading hash not supported

[sodatea]

• Operating System: macOS High Sierra 10.13.6
• Node Version: v10.9.0
• NPM Version: 6.4.0
• webpack version: 4.17.1
• webpack-dev-middleware Version: 3.1.3

• This is a feature request
• This is a bug

For Bugs; How can we reproduce the behavior?

There are several bug reports in vue-cli repository related to this bug:

• BaseURI decoding problem when running serve vuejs/vue-cli#2342
• Nested template literals error vuejs/vue-cli#2345
• URIError: Failed to decode param '/%3C%=%20BASE_URL%20%%3Efavicon.ico' vuejs/vue-cli#2357

To reproduce it, run the following commands:

mkdir "#leadinghash"
cd "#leadinghash"
npx @vue/cli create app --default
cd app
npm run serve -- --open

Expected Behavior

A hello world page.

Actual Behavior

A blank page with following error message logged in terminal:

URIError: Failed to decode param '/%3C%=%20BASE_URL%20%%3Efavicon.ico'

Root Cause

It is actually a bug of url-join(used in middleware.js & util.js):
jfromaniello/url-join#10

require('url-join').('/some/path/with/a/leading/#hash', 'index.html')
// returns '/some/path/with/a/leading#hash/index.html', the `/` before `#` is missing

But I think it is better to solve this issue here because:

1. Maintainers of url-join does not seem to have a strong willingness to fix this long-standing issue;
2. Since url-join is only used to join filesystem paths in webpack-dev-middleware, path.posix,join could be a sufficient replacement.