build: Add --enable-c++20 option

Author: MarcoFalke

ci/test/00_setup_env_native_asan.sh

@@ -11,4 +11,4 @@ export PACKAGES="clang llvm python3-zmq qtbase5-dev qttools5-dev-tools libevent-
 export DOCKER_NAME_TAG=ubuntu:22.04
 export NO_DEPENDS=1
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-zmq --with-incompatible-bdb --with-gui=qt5 CPPFLAGS='-DARENA_DEBUG -DDEBUG_LOCKORDER' --with-sanitizers=address,integer,undefined CC=clang CXX=clang++"
+export BITCOIN_CONFIG="--enable-c++20 --enable-zmq --with-incompatible-bdb --with-gui=qt5 CPPFLAGS='-DARENA_DEBUG -DDEBUG_LOCKORDER' --with-sanitizers=address,integer,undefined CC=clang CXX=clang++"

configure.ac

@@ -78,8 +78,18 @@ AC_ARG_WITH([seccomp],
   [seccomp_found=$withval],
   [seccomp_found=auto])
 
+AC_ARG_ENABLE([c++20],
+  [AS_HELP_STRING([--enable-c++20],
+  [enable compilation in c++20 mode (disabled by default)])],
+  [use_cxx20=$enableval],
+  [use_cxx20=no])
+
 dnl Require C++17 compiler (no GNU extensions)
+if test "$use_cxx20" = "no"; then
 AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
+else
+AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
+fi
 
 dnl Check if -latomic is required for <std::atomic>
 CHECK_ATOMIC