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

fa6e50d6c79633e22ad4cfc75f56aaa40112ecbb fuzz: Use C++20 starts_with in rpc.cpp (MarcoFalke)
faa48388bca06df1ca7ab92461b76a6720481e45 Revert "tracepoints: Disables `-Wgnu-zero-variadic-macro-arguments` to compile without warnings" (MarcoFalke)
fae3b77a87f4d799aca5907335a9dcbab3a51db6 refactor: Drop unused _Pragma to ignore -Wgnu-zero-variadic-macro-arguments (MarcoFalke)
fa02fc0a86c410f907de4fee91dd045547ea4b6e refactor: modernize-use-default-member-init for bit-fields (C++20) (MarcoFalke)
fa67f096bdea9db59dd20c470c9e32f3dac5be94 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 https://github.com/bitcoin/bitcoin/issues/23363 and https://en.cppreference.com/w/cpp/compiler_support#cpp20

  With g++-10 (https://github.com/bitcoin/bitcoin/pull/28348) and clang-13 (https://github.com/bitcoin/bitcoin/pull/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 fa6e50d6c79633e22ad4cfc75f56aaa40112ecbb

Tree-SHA512: 244d79bfb0b750a4bdd713f40573b9ca33816fb84b6c84a58f027b9d7d4bb0cc4f18642959e4cf3d094808a69e5b8a327ca8521d7c0c08af27dacb5da3e78e71

5 files changed, 19 insertions, 34 deletions

diff --git a/src/.clang-format b/src/.clang-format
index 791b3b8f9f..2e5d5c6449 100644
--- a/src/.clang-format
+++ b/src/.clang-format
@@ -43,5 +43,5 @@ SpacesInAngles:  false
 SpacesInContainerLiterals: true
 SpacesInCStyleCastParentheses: false
 SpacesInParentheses: false
-Standard:        c++17
+Standard: c++20
 UseTab:          Never
diff --git a/src/test/fuzz/fuzz.h b/src/test/fuzz/fuzz.h
index 1f0fa5527a..ca74d53de7 100644
--- a/src/test/fuzz/fuzz.h
+++ b/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);                                          \
diff --git a/src/test/fuzz/rpc.cpp b/src/test/fuzz/rpc.cpp
index 2189dc0067..2325bf0941 100644
--- a/src/test/fuzz/rpc.cpp
+++ b/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);
         }
diff --git a/src/txrequest.cpp b/src/txrequest.cpp
index b9a41f26ff..ce5fbd9a7f 100644
--- a/src/txrequest.cpp
+++ b/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.
diff --git a/src/util/trace.h b/src/util/trace.h
index 1fe743f043..b7c275f19b 100644
--- a/src/util/trace.h
+++ b/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