[build] Switch to C++2a

We are unable to switch to C++20 until
https://github.com/mesonbuild/meson/issues/8084 is fixed.

apps/WorkerSleepExample.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Schmaus
 #include <chrono>		 // for milliseconds, operator+, hig...
+#include <compare>	 // for operator<, strong_ordering
 #include <cstdlib>	 // for exit, EXIT_SUCCESS
 #include <iostream>	 // for operator<<, basic_ostream, endl
 #include <ratio>		 // for ratio
@@ -25,6 +26,9 @@ static void letsGetBusy(std::chrono::duration<Rep, Period> duration) {
 			std::chrono::high_resolution_clock::now();
 	const std::chrono::time_point<std::chrono::high_resolution_clock> deadline = now + duration;
 
+	// TODO: The suppressed linter error below may be a false positive
+	// reported by clang-tidy.
+	// NOLINTNEXTLINE(modernize-use-nullptr)
 	while (std::chrono::high_resolution_clock::now() < deadline)
 		;
 }

emper/Scheduler.hpp

@@ -2,7 +2,6 @@
 // Copyright © 2020 Florian Schmaus
 #pragma once
 
-#include <algorithm>	 // for copy, copy_backward
 #include <functional>	 // for function
 
 #include "CallerEnvironment.hpp"

meson.build

@@ -3,7 +3,7 @@ project('EMPER', 'c', 'cpp',
 		default_options : [
 		  'warning_level=3',
 		  'c_std=gnu11',
-		  'cpp_std=c++17',
+		  'cpp_std=c++2a',
 		  'b_ndebug=if-release',
 		  'werror=true',
 		])

tests/AlarmActorTest.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Fischer
-#include <algorithm>	 // for copy
 #include <cstdlib>		 // for exit, EXIT_FAILURE, EXIT_SUC...
 #include <functional>	 // bind
 #include <iostream>		 // for operator<<, basic_ostream

tests/SimpleActorTest.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Schmaus
-#include <algorithm>	 // for copy
 #include <atomic>			 // for atomic_thread_fence, memory_...
 #include <cstdint>		 // for uint64_t
 #include <cstdlib>		 // for exit, EXIT_FAILURE, EXIT_SUC...
@@ -27,7 +26,7 @@ class SumActor : public Actor<uint64_t> {
 	SumActor(Runtime& runtime) : Actor(runtime) {}
 
 	[[nodiscard]] auto getSum() const -> uint64_t {
-		std::atomic_thread_fence(std::memory_order::memory_order_acquire);
+		std::atomic_thread_fence(std::memory_order_acquire);
 		return sum;
 	}
 

tests/TellActorFromAnywhereTest.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
 // Copyright © 2020 Florian Schmaus
-#include <algorithm>	 // for copy, copy_backward
 #include <cstdlib>		 // for exit, EXIT_FAILURE, EXIT_SUCCESS
 #include <functional>	 // for bind
 #include <thread>