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

  With g++-10 (bitcoin/bitcoin#28348) and clang-13 (bitcoin/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

.github/workflows/ci.yml

@@ -62,12 +62,12 @@ jobs:
           echo "TEST_BASE=$(git rev-list -n$((${{ env.MAX_COUNT }} + 1)) --reverse HEAD ^$(git rev-list -n1 --merges HEAD)^@ | head -1)" >> "$GITHUB_ENV"
       - run: |
           sudo apt-get update
-          sudo apt-get install clang ccache build-essential libtool autotools-dev automake pkg-config bsdmainutils python3-zmq libevent-dev libboost-dev libsqlite3-dev libdb++-dev systemtap-sdt-dev libminiupnpc-dev libnatpmp-dev libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools qtwayland5 libqrencode-dev -y
+          sudo apt-get install clang-15 ccache build-essential libtool autotools-dev automake pkg-config bsdmainutils python3-zmq libevent-dev libboost-dev libsqlite3-dev libdb++-dev systemtap-sdt-dev libminiupnpc-dev libnatpmp-dev libqt5gui5 libqt5core5a libqt5dbus5 qttools5-dev qttools5-dev-tools qtwayland5 libqrencode-dev -y
       - name: Compile and run tests
         run: |
           # Run tests on commits after the last merge commit and before the PR head commit
           # Use clang++, because it is a bit faster and uses less memory than g++
-          git rebase --exec "echo Running test-one-commit on \$( git log -1 ) && ./autogen.sh && CC=clang CXX=clang++ ./configure && make clean && make -j $(nproc) check && ./test/functional/test_runner.py -j $(( $(nproc) * 2 ))" ${{ env.TEST_BASE }}
+          git rebase --exec "echo Running test-one-commit on \$( git log -1 ) && ./autogen.sh && CC=clang-15 CXX=clang++-15 ./configure && make clean && make -j $(nproc) check && ./test/functional/test_runner.py -j $(( $(nproc) * 2 ))" ${{ env.TEST_BASE }}
 
   macos-native-x86_64:
     name: 'macOS 13 native, x86_64, no depends, sqlite only, gui'

build-aux/m4/ax_cxx_compile_stdcxx.m4

@@ -983,7 +983,7 @@ 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

@@ -96,18 +96,8 @@ 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=$enableval],
-  [use_cxx20=no])
-
-dnl Require C++17 compiler (no GNU extensions)
-if test "$use_cxx20" = "no"; then
-AX_CXX_COMPILE_STDCXX([17], [noext], [mandatory])
-else
+dnl Require C++20 compiler (no GNU extensions)
 AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
-fi
 
 dnl Unless the user specified OBJCXX, force it to be the same as CXX. This ensures
 dnl that we get the same -std flags for both.

depends/Makefile

@@ -49,7 +49,7 @@ NO_HARDEN ?=
 FALLBACK_DOWNLOAD_PATH ?= https://bitcoincore.org/depends-sources
 
 C_STANDARD ?= c11
-CXX_STANDARD ?= c++17
+CXX_STANDARD ?= c++20
 
 BUILD = $(shell ./config.guess)
 HOST ?= $(BUILD)

depends/README.md

@@ -98,7 +98,7 @@ The following can be set when running make: `make FOO=bar`
 - `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++17`.
+- `CXX_STANDARD`: Set the C++ standard version used. Defaults to `c++20`.
 - `NO_BOOST`: Don't download/build/cache Boost
 - `NO_LIBEVENT`: Don't download/build/cache Libevent
 - `NO_QT`: Don't download/build/cache Qt and its dependencies

depends/packages/qt.mk

@@ -41,7 +41,7 @@ $(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

src/.clang-format

@@ -43,5 +43,5 @@ SpacesInAngles:  false
 SpacesInContainerLiterals: true
 SpacesInCStyleCastParentheses: false
 SpacesInParentheses: false
-Standard:        c++17
+Standard: c++20
 UseTab:          Never

src/test/fuzz/fuzz.h

@@ -33,11 +33,7 @@ struct FuzzTargetOptions {
 
 void FuzzFrameworkRegisterTarget(std::string_view name, TypeTestOneInput target, FuzzTargetOptions opts);
 
-#if defined(__clang__)
-#define FUZZ_TARGET(...) _Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wgnu-zero-variadic-macro-arguments\"") DETAIL_FUZZ(__VA_ARGS__) _Pragma("clang diagnostic pop")
-#else
 #define FUZZ_TARGET(...) DETAIL_FUZZ(__VA_ARGS__)
-#endif
 
 #define DETAIL_FUZZ(name, ...)                                                        \
     void name##_fuzz_target(FuzzBufferType);                                          \

src/test/fuzz/rpc.cpp

@@ -380,9 +380,7 @@ FUZZ_TARGET(rpc, .init = initialize_rpc)
         rpc_testing_setup->CallRPC(rpc_command, arguments);
     } catch (const UniValue& json_rpc_error) {
         const std::string error_msg{json_rpc_error.find_value("message").get_str()};
-        // Once c++20 is allowed, starts_with can be used.
-        // if (error_msg.starts_with("Internal bug detected")) {
-        if (0 == error_msg.rfind("Internal bug detected", 0)) {
+        if (error_msg.starts_with("Internal bug detected")) {
             // Only allow the intentional internal bug
             assert(error_msg.find("trigger_internal_bug") != std::string::npos);
         }

src/txrequest.cpp

@@ -73,7 +73,7 @@ struct Announcement {
     const bool m_is_wtxid : 1;
 
     /** What state this announcement is in. */
-    State m_state : 3;
+    State m_state : 3 {State::CANDIDATE_DELAYED};
     State GetState() const { return m_state; }
     void SetState(State state) { m_state = state; }
 
@@ -97,9 +97,9 @@ struct Announcement {
 
     /** Construct a new announcement from scratch, initially in CANDIDATE_DELAYED state. */
     Announcement(const GenTxid& gtxid, NodeId peer, bool preferred, std::chrono::microseconds reqtime,
-        SequenceNumber sequence) :
-        m_txhash(gtxid.GetHash()), m_time(reqtime), m_peer(peer), m_sequence(sequence), m_preferred(preferred),
-        m_is_wtxid{gtxid.IsWtxid()}, m_state{State::CANDIDATE_DELAYED} {}
+                 SequenceNumber sequence)
+        : m_txhash(gtxid.GetHash()), m_time(reqtime), m_peer(peer), m_sequence(sequence), m_preferred(preferred),
+          m_is_wtxid{gtxid.IsWtxid()} {}
 };
 
 //! Type alias for priorities.

src/util/trace.h

@@ -13,28 +13,19 @@
 
 #include <sys/sdt.h>
 
-// Disabling this warning can be removed once we switch to C++20
-#if defined(__clang__) && __cplusplus < 202002L
-#define BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH _Pragma("clang diagnostic push") _Pragma("clang diagnostic ignored \"-Wgnu-zero-variadic-macro-arguments\"")
-#define BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP _Pragma("clang diagnostic pop")
-#else
-#define BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH
-#define BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#endif
-
-#define TRACE(context, event) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE(context, event) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE1(context, event, a) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE1(context, event, a) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE2(context, event, a, b) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE2(context, event, a, b) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE3(context, event, a, b, c) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE3(context, event, a, b, c) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE4(context, event, a, b, c, d) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE4(context, event, a, b, c, d) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE5(context, event, a, b, c, d, e) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE5(context, event, a, b, c, d, e) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE6(context, event, a, b, c, d, e, f) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE6(context, event, a, b, c, d, e, f) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE7(context, event, a, b, c, d, e, f, g) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE7(context, event, a, b, c, d, e, f, g) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE8(context, event, a, b, c, d, e, f, g, h) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE8(context, event, a, b, c, d, e, f, g, h) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE9(context, event, a, b, c, d, e, f, g, h, i) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE9(context, event, a, b, c, d, e, f, g, h, i) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE10(context, event, a, b, c, d, e, f, g, h, i, j) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE10(context, event, a, b, c, d, e, f, g, h, i, j) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE11(context, event, a, b, c, d, e, f, g, h, i, j, k) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE11(context, event, a, b, c, d, e, f, g, h, i, j, k) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
-#define TRACE12(context, event, a, b, c, d, e, f, g, h, i, j, k, l) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_PUSH DTRACE_PROBE12(context, event, a, b, c, d, e, f, g, h, i, j, k, l) BITCOIN_DISABLE_WARN_ZERO_VARIADIC_POP
+#define TRACE(context, event) DTRACE_PROBE(context, event)
+#define TRACE1(context, event, a) DTRACE_PROBE1(context, event, a)
+#define TRACE2(context, event, a, b) DTRACE_PROBE2(context, event, a, b)
+#define TRACE3(context, event, a, b, c) DTRACE_PROBE3(context, event, a, b, c)
+#define TRACE4(context, event, a, b, c, d) DTRACE_PROBE4(context, event, a, b, c, d)
+#define TRACE5(context, event, a, b, c, d, e) DTRACE_PROBE5(context, event, a, b, c, d, e)
+#define TRACE6(context, event, a, b, c, d, e, f) DTRACE_PROBE6(context, event, a, b, c, d, e, f)
+#define TRACE7(context, event, a, b, c, d, e, f, g) DTRACE_PROBE7(context, event, a, b, c, d, e, f, g)
+#define TRACE8(context, event, a, b, c, d, e, f, g, h) DTRACE_PROBE8(context, event, a, b, c, d, e, f, g, h)
+#define TRACE9(context, event, a, b, c, d, e, f, g, h, i) DTRACE_PROBE9(context, event, a, b, c, d, e, f, g, h, i)
+#define TRACE10(context, event, a, b, c, d, e, f, g, h, i, j) DTRACE_PROBE10(context, event, a, b, c, d, e, f, g, h, i, j)
+#define TRACE11(context, event, a, b, c, d, e, f, g, h, i, j, k) DTRACE_PROBE11(context, event, a, b, c, d, e, f, g, h, i, j, k)
+#define TRACE12(context, event, a, b, c, d, e, f, g, h, i, j, k, l) DTRACE_PROBE12(context, event, a, b, c, d, e, f, g, h, i, j, k, l)
 
 #else