Merge bitcoin#28349: build: Require C++20 compiler

fa6e50d fuzz: Use C++20 starts_with in rpc.cpp (MarcoFalke)
faa4838 Revert "tracepoints: Disables `-Wgnu-zero-variadic-macro-arguments` to compile without warnings" (MarcoFalke)
fae3b77 refactor: Drop unused _Pragma to ignore -Wgnu-zero-variadic-macro-arguments (MarcoFalke)
fa02fc0 refactor: modernize-use-default-member-init for bit-fields (C++20) (MarcoFalke)
fa67f09 build: Require C++20 compiler (MarcoFalke)

Pull request description:

  C++20 allows to write safer code, because it allows to enforce more stuff at compile time (`constinit`, `conteval`, `constexpr`, `std::span`, ...).

  Also, it allows to write less verbose and easier to understand code (C++ 20 Concepts).

  See bitcoin#23363 and https://en.cppreference.com/w/cpp/compiler_support#cpp20

  With g++-10 (bitcoin#28348) and clang-13 (bitcoin#28210), there is broad support for almost all features of C++20.

  It should be fine to require a C++20 compiler for Bitcoin Core 27.0 in 2024 (next year), not the soon upcoming 26.0 next month.

  This pull request includes three small cleanups to make use of C++20 features. If any issues are detected before or after merge, this should be easy to revert. If no issues arise, it should be fine to make use of more involved C++20 features later on.

ACKs for top commit:
  fanquake:
    ACK fa6e50d

Tree-SHA512: 244d79bfb0b750a4bdd713f40573b9ca33816fb84b6c84a58f027b9d7d4bb0cc4f18642959e4cf3d094808a69e5b8a327ca8521d7c0c08af27dacb5da3e78e71

build-aux/m4/ax_cxx_compile_stdcxx.m4

@@ -986,7 +986,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 202002L
+#elif __cplusplus < 201709L  // Temporary patch on top of upstream to allow g++-10
 
 #error "This is not a C++20 compiler"
 

configure.ac

@@ -66,18 +66,8 @@ case $host in
   ;;
 esac
 
-AC_ARG_ENABLE([c++20],
-  [AS_HELP_STRING([--enable-c++20],
-  [enable compilation in c++20 mode (disabled by default)])],
-  [use_cxx20=$enableval],
-  [use_cxx20=no])
-
-dnl Require C++17 or C++20 compiler (no GNU extensions)
-if test "x$use_cxx20" = xyes; then
-  AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
-else
-  AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
-fi
+dnl Require C++20 compiler (no GNU extensions)
+AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
 
 dnl Check if -latomic is required for <std::atomic>
 CHECK_ATOMIC

depends/Makefile

@@ -43,6 +43,9 @@ NO_NATPMP ?=
 MULTIPROCESS ?=
 FALLBACK_DOWNLOAD_PATH ?= https://bitcoincore.org/depends-sources
 
+C_STANDARD ?= c11
+CXX_STANDARD ?= c++20
+
 BUILD = $(shell ./config.guess)
 HOST ?= $(BUILD)
 PATCHES_PATH = $(BASEDIR)/patches

depends/README.md

@@ -99,6 +99,8 @@ The following can be set when running make: `make FOO=bar`
 - `BASE_CACHE`: Built packages will be placed here
 - `SDK_PATH`: Path where SDKs can be found (used by macOS)
 - `FALLBACK_DOWNLOAD_PATH`: If a source file can't be fetched, try here before giving up
+- `C_STANDARD`: Set the C standard version used. Defaults to `c11`.
+- `CXX_STANDARD`: Set the C++ standard version used. Defaults to `c++20`.
 - `NO_QT`: Don't download/build/cache Qt and its dependencies
 - `NO_QR`: Don't download/build/cache packages needed for enabling qrencode
 - `NO_ZMQ`: Don't download/build/cache packages needed for enabling ZeroMQ

depends/packages/qt.mk

@@ -39,7 +39,7 @@ $(package)_config_opts_release += -silent
 $(package)_config_opts_debug = -debug
 $(package)_config_opts_debug += -optimized-tools
 $(package)_config_opts += -bindir $(build_prefix)/bin
-$(package)_config_opts += -c++std c++17
+$(package)_config_opts += -c++std c++2a
 $(package)_config_opts += -confirm-license
 $(package)_config_opts += -hostprefix $(build_prefix)
 $(package)_config_opts += -no-compile-examples