Do something about the windows compilation warning: C++ designated initializers only available with '-std=c++20' or '-std=gnu++20' [-Wc++20-extensions]

[gregorgorjanc]

On https://win-builder.r-project.org we get

* checking whether package 'RcppTskit' can be installed ... WARNING
Found the following significant warnings:
../inst/include/tskit/tskit/core.h:171:21: warning: C++ designated initializers only available with '-std=c++20' or '-std=gnu++20' [-Wc++20-extensions]

The code causing this is

/*
We define a specific NAN value for default mutation time which indicates
the time is unknown. We use a specific value so that if mutation time is set to
a NAN from a computation we can reject it. This specific value is a non-signalling
NAN with the last six fraction bytes set to the ascii of "tskit!"
*/
#define TSK_UNKNOWN_TIME_HEX 0x7FF874736B697421ULL
static inline double
__tsk_nan_f(void)
{
    const union {
        uint64_t i;
        double f;
    } nan_union = { .i = TSK_UNKNOWN_TIME_HEX };
    return nan_union.f;
}

The .i = is throwing the error, but only on CRAN Windows (not on GitHub windows or R Universe windows) and chat with tskit-devs suggests this is CRAN windows toolset issue tskit-dev/tskit#3375.

This is currently blocking CRAN submission :( #45

[gregorgorjanc]

I tried submission to https://win-builder.r-project.org with:

• Removing the above mentioned .i = and the win-builder compiles without a WARNING, however this would require upstream change of valid C code :(

• Adding CXX_STD = CXX20 to Makevars and the win-builder compiles without a WARNING, so I think I will go with this option at this stage to get over this hurdle and evaluate downstream if this limits distribution of RcppTskit due to specifying C++ 2020 standard (which seems to become broadly used 2021-2023, so some older systems might complain!?)

• A third alternative, that I did not check, seems to be to ignore warnings from tskit/core.h using the following in our shim core.h - not sure if I like or dislike this option ... but is one option if specifying C++ 2020 standard is too limiting.

#ifndef RCPPTSKIT_TSKIT_CORE_H
#define RCPPTSKIT_TSKIT_CORE_H

#if defined(__cplusplus)
  #if defined(__clang__)
    #pragma clang diagnostic push
    #pragma clang diagnostic ignored "-Wc++20-extensions"
  #elif defined(__GNUC__)
    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wc++20-extensions"
  #endif
#endif

#include <tskit/tskit/core.h>

#if defined(__cplusplus)
  #if defined(__clang__)
    #pragma clang diagnostic pop
  #elif defined(__GNUC__)
    #pragma GCC diagnostic pop
  #endif
#endif

/* existing shim content ... */

#endif```

[gregorgorjanc]

Done in #64