Latest macOS system header causes compilation failures on GCC

[morinmorin]

Description
After recent developer tools update on macOS Sonoma (i.e. "Command Line Tools for Xcode 15.3"), some Catch2 headers fail to compile on GCC.

Test case

// v2's `single_include/catch2/catch.hpp` includes `TargetConditionals.h`
#include <catch2/catch.hpp>

and

// v3's `catch2/internal/catch_platform.hpp`, which is included by `catch2/catch_all.hpp`, includes `TargetConditionals.h`
#include <catch2/catch_all.hpp>

fail to compile on GCC 13 or earlier, after updating to "Command Line Tools for Xcode 15.3".

Background
"Command Line Tools for Xcode 15.3" introduces a problematic code at Line 140 of usr/include/TargetConditionals.h (i.e. MacOSX14.4.sdk/usr/include/TargetConditionals.h):

#if !defined(__has_extension) || # !__has_extension(define_target_os_macros)

This is problematic, since compilers without __has_extension support cause compilation failure with it. Notably, GCC does not support __has_extension until the next release (GCC 14).

(I think this should be fixed upstream by Apple, but ...) it may make some sense to add guard for __has_extension:

#ifdef __APPLE__
#  ifndef __has_extension
#  define __has_extension(x) 0
#  endif
#  include <TargetConditionals.h>

[horenmar]

Why does the compilation fail there? Is this a hard error?

[morinmorin]

Yes, that's hard errors. For example, on mac with the latest developer tools, this

echo '#include <catch2/catch_all.hpp>' | g++-13 -xc++ -

produces the following error messages

In file included from catch2/internal/catch_platform.hpp:14,
                 from catch2/internal/catch_compiler_capabilities.hpp:27,
                 from catch2/benchmark/catch_benchmark.hpp:14,
                 from catch2/benchmark/catch_benchmark_all.hpp:24,
                 from catch2/catch_all.hpp:25,
                 from <stdin>:1:
/usr/include/TargetConditionals.h:140:50: error: missing binary operator before token "("
  140 | #if !defined(__has_extension) || !__has_extension(define_target_os_macros)

The code !__has_extension(define_target_os_macros) causes compilation failure, because __has_extension is not supported/defined on GCC (until the next release of GCC).

[horenmar]

Hmmm, yeah there was a reason why we are checking headers like this

#if defined(__has_include)
  #if __has_include(<string_view>) && defined(CATCH_CPP17_OR_GREATER)

I'll accept a PR that adds the workaround.

[morinmorin]

Thanks. I'll submit PR later.