Add tests for C++20 std::u8string

Also, add missing includes:

 #include <system_error>  // for error_code
 #include <type_traits>   // for is_same

 #include <cerrno>        // for errno

Author: MarcoFalke

src/test/fs_tests.cpp

@@ -18,8 +18,10 @@ BOOST_FIXTURE_TEST_SUITE(fs_tests, BasicTestingSetup)
 BOOST_AUTO_TEST_CASE(fsbridge_pathtostring)
 {
     std::string u8_str = "fs_tests_₿_🏃";
+    std::u8string str8{u8"fs_tests_₿_🏃"};
     BOOST_CHECK_EQUAL(fs::PathToString(fs::PathFromString(u8_str)), u8_str);
     BOOST_CHECK_EQUAL(fs::u8path(u8_str).u8string(), u8_str);
+    BOOST_CHECK_EQUAL(fs::path(str8).u8string(), u8_str);
     BOOST_CHECK_EQUAL(fs::PathFromString(u8_str).u8string(), u8_str);
     BOOST_CHECK_EQUAL(fs::PathToString(fs::u8path(u8_str)), u8_str);
 #ifndef WIN32
@@ -47,7 +49,7 @@ BOOST_AUTO_TEST_CASE(fsbridge_fstream)
     fs::path tmpfolder = m_args.GetDataDirBase();
     // tmpfile1 should be the same as tmpfile2
     fs::path tmpfile1 = tmpfolder / fs::u8path("fs_tests_₿_🏃");
-    fs::path tmpfile2 = tmpfolder / fs::u8path("fs_tests_₿_🏃");
+    fs::path tmpfile2 = tmpfolder / fs::path(u8"fs_tests_₿_🏃");
     {
         std::ofstream file{tmpfile1};
         file << "bitcoin";

src/util/fs.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2022 The Bitcoin Core developers
+// Copyright (c) 2017-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -18,6 +18,7 @@
 #endif
 
 #include <cassert>
+#include <cerrno>
 #include <string>
 
 namespace fsbridge {
@@ -130,4 +131,4 @@ std::string get_filesystem_error_message(const fs::filesystem_error& e)
 #endif
 }
 
-} // fsbridge
+} // namespace fsbridge

src/util/fs.h

@@ -1,4 +1,4 @@
-// Copyright (c) 2017-2022 The Bitcoin Core developers
+// Copyright (c) 2017-present The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -14,6 +14,8 @@
 #include <ios>
 #include <ostream>
 #include <string>
+#include <system_error>
+#include <type_traits>
 #include <utility>
 
 /** Filesystem operations and types */