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`std::stod ("nan")` returns negative NaN




Hi

I've found a curious behavior about `std::stod ("nan")` on Cygwin.
Only on Cygwin, `std::stod ("nan")` returns negative NaN.
On Linux etc., `std::stod ("nan")` returns positive NaN.

Here is a reproduction code.

```
// g++ -std=c++11 foobar.cc

#include <iostream>
#include <limits>
#include <string>

int main ()
{
  std::cout << "stod (\"nan\") = "
            << std::stod ("nan")
            << std::endl;
  std::cout << "stod (\"-nan\") = "
            << std::stod ("-nan")
            << std::endl;

  std::cout << "quiet_NaN () = "
            << std::numeric_limits<double>::quiet_NaN ()
            << std::endl;
}
```

The result on Cygwin 2.10.0 64 bit (g++ 7.3.0):
```
stod ("nan") = -nan
stod ("-nan") = nan
quiet_NaN () = nan
```

The result on MinGW-w64 64 bit (g++ 4.9.2):
```
stod ("nan") = nan
stod ("-nan") = nan
quiet_NaN () = nan
```

The result on Ubuntu 16.04 LTS 64 bit (g++ 5.4.0):
```
stod ("nan") = nan
stod ("-nan") = nan
quiet_NaN () = nan
```

The result on FreeBSD 10.1 64 bit (clang++ 3.4.1):
```
stod ("nan") = nan
stod ("-nan") = nan
quiet_NaN () = nan
```

Is it correct that returning negative NaN on Cygwin?

Thanks.

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