[Fletchgen||C++] Builds with Clang fail since Arrow 0.17 upgrade

[johanpel]

It seems that upgrading to 0.17 causes Clang to fail building Fletchgen, while GCC succeeds.

Initially I get stuff like:

fletcher/common/cpp/src/fletcher/arrow-utils.cc:164:3: error: ignoring return value of function declared with 'warn_unused_result' attribute [-Werror,-Wunused-result]
  fis->Close();

In an attempt to fix this it seems that this code in Arrow from arrow/result.h

#if __cplusplus >= 201703L
using std::launder;
#else
template <class T>
constexpr T* launder(T* p) noexcept {
  return p;
}
#endif

causes additional problems, since std::launder isn't there.

It's probably some discrepancy between GCC and Clang concerning that launder library in C++17, whatever that thing is.

gcc (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0 clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)

[mbrobbel]

@johanpel With

auto status = fis->Close();

it compiles for me. (However, this obviously still ignores the status)

[johanpel]

Seems to work fine now with Clang 10. I will close this, we can reopen if it poses any issues for anyone else.