GHC inconsistently handles \\?\ for long paths on Windows

To demonstrate what I mean first:

>ghc-pkg init \\?\c:\Users\michael\Desktop\some-pkg-db
>dir \\?\c:\Users\Michael\Desktop\some-pkg-db
 Volume in drive \\?\c: is Windows8_OS
 Volume Serial Number is CEF7-874D

 Directory of \\?\c:\Users\Michael\Desktop\some-pkg-db

09/01/2015  09:38 AM    <DIR>          .
09/01/2015  09:38 AM    <DIR>          ..
09/01/2015  09:38 AM                40 package.cache
               1 File(s)             40 bytes
               2 Dir(s)   5,907,509,248 bytes free
>ghc -package-db \\?\c:\Users\Michael\Desktop\some-pkg-db
ghc: can't find a package database at \\?\c:\Users\Michael\Desktop\some-pkg-db

The problem: on Windows by default, paths are limited to 260 characters, which can become a real problem in practice when dealing with the standard library path hierarchy the Cabal library uses and any package with a decent name length. This was a real problem in practice, see:

https://github.com/haskell/cabal/issues/2502

Windows has a workaround for this: you can prefix an absolute path with \\?\, see:

https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247%28v=vs.85%29.aspx#maxpath

It would be very useful if such paths worked with GHC, but currently they do not. I don't yet know how deep the problem is.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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