Sv: g++ and c++17 filesystem

[Eliot Moss]

Ok, first, I admit that I was not familiar with the details of std::filesystem.  However, after 
looking at it, I remain unsurprised that the Cygwin and Mingw versions might be different.  (I would 
also not be surprised if there is a real bug in there.)  The behavior I would _expect_ is that the 
Cygwin version will work using Posix sorts of assumptions.  While a root of C: (for example) _might_ 
work, /cygdrive/c is more normative on Cygwin.  (I put a link to that in Cygwin's / called c, so 
that, for me, /c works.)  Likewise, I would expect the normative path separator to be / not \, and 
an absolute path to start with /.  Windows offers several kinds of symlinks, with varying semantics, 
so the detailed behavior of that would be affected by the settings in the CYGWIN environment variable.

I would expect std::filesystem to present paths to construct paths to present to underlying library 
calls such as open ... and on Cygwin, open uses Posix style paths.

I "get" that you want to write portable programs that use this interface, which is analogous to the 
Java file path classes.  In terms of how this interface works, I would expect it to _claim_ that it 
is Posix, not Windows, because the paths Cygwin supports are Posix style (it _will_ recognize a few 
Windows idioms, but it is correct in not advertising itself as Windows).

So it you want to do Windows-style (but abstracted with this library), I direct you to Mingw.  Each 
has its place.  Cygwin allows one to pretend, pretty successfully though with a few small rough 
edges, that one is on Linux, not Windows.  That is its intent.  Mingw gives you the gcc/gnu 
toolchain and libraries under Windows.

I hope we're not still talking at cross purposes, though that it certainly possible!

Best wishes - EM