fs: work around u8path deprecated-declaration warnings with libc++ #25808

When building in c++20 mode using libc++, the following warning is emitted:

./fs.h:72:29: warning: 'u8path<std::string>' is deprecated [-Wdeprecated-declarations]
    return std::filesystem::u8path(utf8_str);
                            ^
/usr/lib/llvm-14/bin/../include/c++/v1/__filesystem/u8path.h:72:27: note: 'u8path<std::string>' has been explicitly marked deprecated here
_LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_WITH_CHAR8_T
                          ^
/usr/lib/llvm-14/bin/../include/c++/v1/__config:1042:43: note: expanded from macro '_LIBCPP_DEPRECATED_WITH_CHAR8_T'
                                          ^
/usr/lib/llvm-14/bin/../include/c++/v1/__config:1007:48: note: expanded from macro '_LIBCPP_DEPRECATED'
                                               ^
1 warning generated.

as u8path<std::string> is deprecated starting with C++20.

Fixes: #24682.

tACK 6bf6234790099cce5585b62742ffb572054973e1

Although I don't see any other deprecation warnings, I like that we only suppress this specific one.

I do prefer @MarcoFalke's alternative which also allows to drop the global _SILENCE_CXX20_U8PATH_DEPRECATION_WARNING:

diff --git a/build_msvc/common.init.vcxproj.in b/build_msvc/common.init.vcxproj.in
index f7169a346d..e84ac03623 100644
--- a/build_msvc/common.init.vcxproj.in
+++ b/build_msvc/common.init.vcxproj.in
@@ -90,7 +90,7 @@
       <AdditionalOptions>/utf-8 /Zc:__cplusplus /std:c++20 %(AdditionalOptions)</AdditionalOptions>
       <DisableSpecificWarnings>4018;4244;4267;4334;4715;4805;4834</DisableSpecificWarnings>
       <TreatWarningAsError>true</TreatWarningAsError>
-      <PreprocessorDefinitions>_SILENCE_CXX20_U8PATH_DEPRECATION_WARNING;_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING;_SILENCE_CXX17_OLD_ALLOCATOR_MEMBERS_DEPRECATION_WARNING;ZMQ_STATIC;NOMINMAX;WIN32;HAVE_CONFIG_H;_CRT_SECURE_NO_WARNINGS;_SCL_SECURE_NO_WARNINGS;_CONSOLE;_WIN32_WINNT=0x0601;_WIN32_IE=0x0501;WIN32_LEAN_AND_MEAN;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING;_SILENCE_CXX17_OLD_ALLOCATOR_MEMBERS_DEPRECATION_WARNING;ZMQ_STATIC;NOMINMAX;WIN32;HAVE_CONFIG_H;_CRT_SECURE_NO_WARNINGS;_SCL_SECURE_NO_WARNINGS;_CONSOLE;_WIN32_WINNT=0x0601;_WIN32_IE=0x0501;WIN32_LEAN_AND_MEAN;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <AdditionalIncludeDirectories>..\..\src;..\..\src\minisketch\include;..\..\src\univalue\include;..\..\src\secp256k1\include;..\..\src\leveldb\include;..\..\src\leveldb\helpers\memenv;%(AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
     </ClCompile>
     <Link>
diff --git a/src/fs.h b/src/fs.h
index e8b34319bb..ac58c4a2ba 100644
--- a/src/fs.h
+++ b/src/fs.h
@@ -69,7 +69,11 @@ public:
 
 static inline path u8path(const std::string& utf8_str)
 {
+#if __cplusplus < 202002L
     return std::filesystem::u8path(utf8_str);
+#else
+    return std::filesystem::path(std::u8string{utf8_str.begin(), utf8_str.end()});
+#endif
 }
 
 // Disallow implicit std::string conversion for absolute to avoid

I do prefer @MarcoFalke's #24682 (comment)

I don't really want to start introducing __cplusplus version checks into our code.

which also allows to drop the global _SILENCE_CXX20_U8PATH_DEPRECATION_WARNING:

Why does dropping that matter? It's being used as intended.

No opinion on what to do, but the new code should ideally be the version we'll adopt once (and if) we bump to C++20

Code review ACK 6bf6234790099cce5585b62742ffb572054973e1. I do think hebasto/marco's solution #25808 (comment) is cleaner, more standard, and more future proof but both solutions are good.

I'm still not a fan of the version checking, but I've pushed up the alternative solution.

review ACK ced00f5a2e2b8623d1ee6a9afef80fc5df08d509

Looks like this creates 3 copies of the string, but they should all be moved so it has probably no performance impact. Regardless, the helper shouldn't be used in hot loops anyway.

ACK ced00f5a2e2b8623d1ee6a9afef80fc5df08d509