"I mean C:\\temp is an invalid path under Windows."

While I'm not disagreeing with that absolute statement, C:\\temp may be required under a Windows environment. I came across an example of this fairly recently.

Consider a pp command on cmd.exe something like this (using Strawberry Perl):

C:\some\dir>pp @long_list_of_options.txt -o filename.exe filename.pl
[download]

Options (in long_list_of_options.txt) which include paths cannot be written like this for example:

--link=C:\path\to\some.dll
[download]

The parts like \X will be interpreted as backslashed escapes: resulting in just X, or special characters (e.g. newline, tab, and so on). The example option I showed would end up like:

--link=C:path    osome.dll
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In order to get it to resolve to the example shown, you'd need to write:

--link=C:\\path\\to\\some.dll
[download]

That doesn't mean that an XY Problem doesn't exist; it just shows that it might not exist. Having said that, the terminal \\\\ certainly looks highly dodgy.