[MacPorts] #62426: libc++: using a newer libc++ to build software on older macos systems

[MacPorts]

#62426: libc++: using a newer libc++ to build software on older macos systems
--------------------------+----------------------
  Reporter:  kencu        |      Owner:  kencu
      Type:  enhancement  |     Status:  assigned
  Priority:  Normal       |  Milestone:
 Component:  ports        |    Version:
Resolution:               |   Keywords:
      Port:  libcxx       |
--------------------------+----------------------

Comment (by kencu):

 For a start, we'll try to build the attached cpp file that uses the new
 std::filesystem features.

 We'll do this on Mojave, which does not have these features in the OS-
 supplied libc++.dylib.

 Using the os-supplied Xcode clang, we don't get far:
 {{{
 $ /usr/bin/clang++ fstest.cpp
 fstest.cpp:9:21: error: expected namespace name
 namespace fs = std::filesystem;
                ~~~~~^
 fstest.cpp:12:28: error: use of undeclared identifier 'fs'
 void DisplayPathInfo(const fs::path& pathToShow)
                            ^
 fstest.cpp:16:13: warning: 'auto' type specifier is a C++11 extension
 [-Wc++11-extensions]
         for (const auto& part : pathToShow)
                    ^
 ... and more
 }}}

 of course, we need c++17 for this to work so we add that, but that gives
 us different errors:
 {{{
 $ /usr/bin/clang++ -std=c++17 fstest.cpp
 fstest.cpp:12:32: error: 'path' is unavailable: introduced in macOS 10.15
 void DisplayPathInfo(const fs::path& pathToShow)
                                ^
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/filesystem:739:24:
 note: 'path' has been
       explicitly marked unavailable here
 class _LIBCPP_TYPE_VIS path {
                        ^
 fstest.cpp:15:39: error: 'operator<<' is unavailable: introduced in macOS
 10.15
         cout << "Displaying path info for: " << pathToShow << "\n";
                                              ^
 ... and more
 }}}

 Apple has marked these {{{std::filesystem}}} features as not being
 available in the system libc++.dylib by blocking them in the clang c++
 headers, with this file:
 {{{
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__config
 }}}

 This can be overridden by passing a specific define:
 {{{
 // Decide whether to use availability macros.
 #if !defined(_LIBCPP_BUILDING_LIBRARY) &&
 \
     !defined(_LIBCXXABI_BUILDING_LIBRARY) &&
 \
     !defined(_LIBCPP_DISABLE_AVAILABILITY) &&
 \
     __has_feature(attribute_availability_with_strict) &&
 \
     __has_feature(attribute_availability_in_templates) &&
 \
     __has_extension(pragma_clang_attribute_external_declaration)
 #  ifdef __APPLE__
 #    define _LIBCPP_USE_AVAILABILITY_APPLE
 #  endif
 #endif
 }}}

 and so we add that define to disable the availability tests, and then we
 actually see that the link fails because the symbols are not in
 {{{/usr/lib/libc++.dylib}}}
 {{{
 $ /usr/bin/clang++ -std=c++17 -D_LIBCPP_DISABLE_AVAILABILITY fstest.cpp
 Undefined symbols for architecture x86_64:
   "std::__1::__fs::filesystem::path::__filename() const", referenced from:
       std::__1::__fs::filesystem::path::filename() const in fstest-
 049dac.o
       std::__1::__fs::filesystem::path::has_filename() const in fstest-
 049dac.o
 .. and more ...
 }}}

-- 
Ticket URL: <https://trac.macports.org/ticket/62426#comment:1>
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