DRAFT: Upgrade to C++20

[drebelsky]

Upgrade to C++20. Notes:

• autocheck uses std::result_of (deprecated in 17, removed in 20)—I replaced just the use of that since we seem to have a de-facto in-tree fork of autocheck (e.g., we reference stellar::).

• fmt::format requires a either a compile-time constant first parameter in 20 or a fmt::runtime call.

• for fmt to work on clang, it seems that we need to have a version starting from this commit. Otherwise, code like the following doesn't work

  auto out = std::vector<char>();
  fmt::format_to(std::back_inserter(out), FMT_STRING("{}"), 42);

  So, I bumped the versions of fmt and spdlog

  • With the newer version of spdlog, FMT_STRING without any template arguments no longer picks up the right overload, but the C++20 version seems to correctly do compile-time strings with out it, so I updated Logging.h to reflect this. One alternative is using the following macro, but I think it's probably cleaner to just remove the FMT_STRING usage in Logging.h

  #define ID(ARG) ARG
  #define LPAREN (
  #define RPAREN )
  /* Expands to `FMT_STRING(f), __VA_ARGS__` when __VA_ARGS__ is non-empty and `f`
   * otherwise (spdlog will not pick the proper overload if we only pass
   * FMT_STRING(f)) */
  #define LOG_FMT_STRING(f, ...) \
      ID(__VA_OPT__(FMT_STRING LPAREN) f __VA_OPT__(RPAREN)) \
      __VA_OPT__(, ) __VA_ARGS__

• xdrpp needs to get updated for two reasons.

  1. On its current master, we end up trying to build using C++14, but it should be at least 17. Bumping to 20 (or 17) we get some extra flags¹

     checking whether ccache clang++ -std=c++20 accepts -g... yes
     checking for ccache clang++ -std=c++20 option to enable C++11 features... -std=gnu++11
     checking dependency style of ccache clang++ -std=c++20 -std=gnu++11... gcc3
     checking whether ccache clang++ -std=c++20 -std=gnu++11 supports C++20 features by default... no
     checking whether ccache clang++ -std=c++20 -std=gnu++11 supports C++20 features with -std=gnu++20... yes
     

  2. Because of the spaceship operator, at the very least, we need operator<=> defined for xdr::pointer since it inherits from std::unique_ptr which defines <=>, meaning that without the overload, e.g., std::vector<xdr::pointer<int>> comparisons no longer work properly (they go back to comparing by address instead of value).

  There is the cxx20 branch, but it doesn't have the most recent 5 commits from the master branch (which we rely on in core). Additionally, it seems to be somewhat buggy (e.g., tests/marshal.cc doesn't pass). So, for the purposes of this PR, I think it's probably easiest to just bump to 20 and add operator<=> for xdr data types.

• Small change, but renamed the constructor in the header file for ScopedLedgerEntry since ScopedLedgerEntry<T> isn't technically valid since DR 2237 was adopted in C++20 (link).

Footnotes

1. Due to a known issue: autoconf adds a flag to enable C++11 features since C++20 isn't fully backwards compatible. Unfortunately, the patch to fix this behavior is not in a released version of autoconf ↩

[drebelsky]

Looking at it now, I see we had some custom overrides in autocheck (e.g., using stellar:: prefixed random number generators), so I will re-evaluate that (maybe we should fork autocheck?).

[drebelsky]

Looks like this failed last time because of xdrpp building on C++14, bumping to the commit introduced in this pr

[drebelsky]

The test case "LedgerTxnRoot prefetch classic entries" fails because vectors of xdr::pointers break (due to the spaceship operator overloads) in C++20, so I will work on merging the cxx20 branch with master in xdrpp.