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

[Masamichi Hosoda <trueroad at trueroad dot jp>]

>> 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
> ```

If I understand correctly,
`std::stod ()` uses cygwin1.dll's `strtod ()` for the conversion.

`std::stod ()` is defined in
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/bits/basic_string.h L6388-.
It calls `__gnu_cxx::__stoa ()` with pointer of `std::strtod ()`.

```
  inline double
  stod(const string& __str, size_t* __idx = 0)
  { return __gnu_cxx::__stoa(&std::strtod, "stod", __str.c_str(), __idx); }
```

`__gnu_cxx::__stoa ()` is defined in
/usr/lib/gcc/x86_64-pc-cygwin/7.3.0/include/c++/ext/string_conversions.h L51-.
It calls the first parameter, `std::strtod ()`.

`std::strtod ()` is cygwin1.dll's `strtod ()`.

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