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FWIW I understand the backslash should be percent-encoded in URLs, otherwise the URL is not valid.
This reminds me of a few other bugs:
* Issue 30500: Made the behaviour of fragment (#. . .) versus userinfo (. . .@) consistent, e.g. in //www.google.com#@xxx.com
* Issue 18140: Also about the ambiguity of fragment (#. . .) and query (?. . .) versus userinfo (. . .@)
* Issue 23328: Precedence of path segment (/. . .) versus userinfo (. . .@); e.g. //user/name:pass/word@www.google.com
I think people some times come up with these invalid URLs because they are trying to make a URL that includes a password with unusual characters (e.g. for the "http_proxy" environment variable). So raising an exception or otherwise changing the parsing behaviour could break those cases.