[libc++][print] Moves is_terminal to the dylib.

Having the test in the header requires including unistd.h on POSIX
platforms. This header has other declarations which may conflict with
code that uses named declarations provided by this header. For example
code using "int pipe;" would conflict with the function pipe in this
header.

Moving the code to the dylib means std::print would not be available on
Apple backdeployment targets. On POSIX platforms there is no transcoding
required so a not Standard conforming implementation is still a useful
and the observable differences are minimal. This behaviour has been done
for print before #76293.

Note questions have been raised in LWG4044 "Confusing requirements for
std::print on POSIX platforms", whether or not the isatty check on POSIX
platforms is required. When this LWG issue is resolved the
backdeployment targets could become Standard compliant.

This patch is intended to be backported to the LLVM-18 branch.

Fixes: #79782

Author: mordante

libcxx/include/print

@@ -32,6 +32,7 @@ namespace std {
 */
 
 #include <__assert> // all public C++ headers provide the assertion handler
+#include <__availability>
 #include <__concepts/same_as.h>
 #include <__config>
 #include <__system_error/system_error.h>
@@ -43,10 +44,6 @@ namespace std {
 #include <string_view>
 #include <version>
 
-#if __has_include(<unistd.h>)
-#  include <unistd.h>
-#endif
-
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
@@ -68,7 +65,8 @@ _LIBCPP_EXPORTED_FROM_ABI bool __is_windows_terminal(FILE* __stream);
 // Note the function is only implemented on the Windows platform.
 _LIBCPP_EXPORTED_FROM_ABI void __write_to_windows_console(FILE* __stream, wstring_view __view);
 #  endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
-
+#elif __has_include(<unistd.h>)
+_LIBCPP_EXPORTED_FROM_ABI bool __is_posix_terminal(FILE* __stream);
 #endif // _LIBCPP_WIN32API
 
 #if _LIBCPP_STD_VER >= 23
@@ -200,10 +198,12 @@ _LIBCPP_HIDE_FROM_ABI inline bool __is_terminal(FILE* __stream) {
   // the behavior in the test. This is not part of the public API.
 #  ifdef _LIBCPP_TESTING_PRINT_IS_TERMINAL
   return _LIBCPP_TESTING_PRINT_IS_TERMINAL(__stream);
+#  elif _LIBCPP_AVAILABILITY_HAS_PRINT == 0
+  return false;
 #  elif defined(_LIBCPP_WIN32API)
   return std::__is_windows_terminal(__stream);
 #  elif __has_include(<unistd.h>)
-  return isatty(fileno(__stream));
+  return std::__is_posix_terminal(__stream);
 #  else
 #    error "Provide a way to determine whether a FILE* is a terminal"
 #  endif

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.abilist

@@ -1188,6 +1188,7 @@
 {'is_defined': True, 'name': '_ZNSt3__118shared_timed_mutex8try_lockEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__118shared_timed_mutexC1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__118shared_timed_mutexC2Ev', 'type': 'FUNC'}
+{'is_defined': True, 'name': '_ZNSt3__119__is_posix_terminalEP8_IO_FILE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__119__shared_mutex_base11lock_sharedEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__119__shared_mutex_base13unlock_sharedEv', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__119__shared_mutex_base15try_lock_sharedEv', 'type': 'FUNC'}

libcxx/src/print.cpp

@@ -8,22 +8,26 @@
 
 #include <__config>
 
-#if defined(_LIBCPP_WIN32API)
+#include <cstdlib>
+#include <print>
+
+#include <__system_error/system_error.h>
 
-#  include <cstdlib>
-#  include <print>
+#include "filesystem/error.h"
 
+#if defined(_LIBCPP_WIN32API)
 #  define WIN32_LEAN_AND_MEAN
 #  define NOMINMAX
 #  include <io.h>
 #  include <windows.h>
-
-#  include <__system_error/system_error.h>
-
-#  include "filesystem/error.h"
+#elif __has_include(<unistd.h>)
+#  include <unistd.h>
+#endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if defined(_LIBCPP_WIN32API)
+
 _LIBCPP_EXPORTED_FROM_ABI bool __is_windows_terminal(FILE* __stream) {
   // Note the Standard does this in one call, but it's unclear whether
   // an invalid handle is allowed when calling GetConsoleMode.
@@ -52,6 +56,9 @@ __write_to_windows_console([[maybe_unused]] FILE* __stream, [[maybe_unused]] wst
 }
 #  endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS
 
-_LIBCPP_END_NAMESPACE_STD
+#elif __has_include(<unistd.h>) // !_LIBCPP_WIN32API
 
-#endif // !_LIBCPP_WIN32API
+_LIBCPP_EXPORTED_FROM_ABI bool __is_posix_terminal(FILE* __stream) { return isatty(fileno(__stream)); }
+#endif
+
+_LIBCPP_END_NAMESPACE_STD