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

[Masamichi Hosoda]

> On Mon, 13 Aug 2018 at 19:46, Duncan Roe <duncan_roe@optusnet.com.au> wrote:
>>
>> On Mon, Aug 13, 2018 at 12:52:48PM -0400, Stephen John Smoogen wrote:
>> > On Mon, 13 Aug 2018 at 11:16, Masamichi Hosoda <trueroad@trueroad.jp> wrote:
>> [...]
>> > On Fedora 27 with 7.3.1 it gives
>> > ```
>> > stod ("nan") = nan
>> > stod ("-nan") = nan
>> > quiet_NaN () = nan
>> > ```
>> [...]
> 
> I tested a compile on Windows Subsystem for Linux Ubuntu 18.04 which
> had gcc 7.3.0. The output was the same as everything but Cygwin. At
> this point it is time to pull through the gdb debugger and try to
> figure out where it is coming from.

I suspect `cygwin1.dll`'s `strtod ()` etc.
Here's a code.

```
/* gcc foobar.c */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int main (void)
{
  printf ("strtod (\"nan\", NULL) = %f\n", strtod ("nan", NULL));
  printf ("strtod (\"-nan\", NULL) = %f\n", strtod ("-nan", NULL));
  printf ("nan (\"\") = %f\n", nan (""));

  return 0;
}
```

Cygwin 2.10.0 64 bit with gcc 7.3.0
```
strtod ("nan", NULL) = -nan
strtod ("-nan", NULL) = nan
nan ("") = nan
```

Ubuntu 16.04 LTS 64 bit with gcc 5.4.0
```
strtod ("nan", NULL) = nan
strtod ("-nan", NULL) = nan
nan ("") = nan
```

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