[libc++] Improve the detection of whether the blocks runtime is available

[ldionne]

Summary

The runtime for Blocks may not be available even though the Blocks
language extension _is_ available. Instead of potentially failing,
this commit is much more conservative and assumes the runtime for
Blocks is only provided on Apple platforms.

[davezarzycki]

LGTM

[compnerd]

We should be able to check for this in CMake, but that can be improved in the future. (Note that the blocks runtime is in compiler-rt, so we should actually be able to build and use that on all targets)

libcxx/include/__config

 #if __has_feature(objc_arc_weak)
 #define _LIBCPP_HAS_OBJC_ARC_WEAK
 #endif
 
 #if __has_extension(blocks)
 #  define _LIBCPP_HAS_EXTENSION_BLOCKS
 #endif
 
+#if defined(_LIBCPP_HAS_EXTENSION_BLOCKS) && defined(__APPLE__)
+#  define _LIBCPP_HAS_BLOCKS_RUNTIME
+#endif
+
 #if !(__has_feature(cxx_relaxed_constexpr))
 #define _LIBCPP_HAS_NO_CXX14_CONSTEXPR
 #endif
 
 #if !(__has_feature(cxx_variable_templates))
 #define _LIBCPP_HAS_NO_VARIABLE_TEMPLATES
 #endif
 

libcxx/include/functional

 #include <exception>
 #include <memory>
 #include <tuple>
 #include <utility>
 #include <version>
 
 #include <__functional_base>
 
-#if defined(_LIBCPP_HAS_EXTENSION_BLOCKS) && !defined(_LIBCPP_HAS_OBJC_ARC) && __has_include(<Block.h>)
+#if defined(_LIBCPP_HAS_BLOCKS_RUNTIME) && !defined(_LIBCPP_HAS_OBJC_ARC)
 #include <Block.h>
 #endif
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
_LIBCPP_INLINE_VISIBILITY const _Tp* target() const _NOEXCEPT
         if (__policy_->__clone) // Out of line storage.
             return reinterpret_cast<const _Tp*>(__buf_.__large);
         else
             return reinterpret_cast<const _Tp*>(&__buf_.__small);
     }
 #endif // _LIBCPP_NO_RTTI
 };
 
-#if defined(_LIBCPP_HAS_EXTENSION_BLOCKS) && !defined(_LIBCPP_HAS_OBJC_ARC) && __has_include(<Block.h>)
+#if defined(_LIBCPP_HAS_BLOCKS_RUNTIME) && !defined(_LIBCPP_HAS_OBJC_ARC)
 
 template<class _Rp1, class ..._ArgTypes1, class _Alloc, class _Rp, class ..._ArgTypes>
 class __func<_Rp1(^)(_ArgTypes1...), _Alloc, _Rp(_ArgTypes...)>
     : public  __base<_Rp(_ArgTypes...)>
 {
     typedef _Rp1(^__block_type)(_ArgTypes1...);
     __block_type __f_;
 

libcxx/test/libcxx/utilities/function.objects/func.blocks.sh.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // std::function support for the "blocks" extension
 
 // UNSUPPORTED: c++98, c++03
-// REQUIRES: has-fblocks
+
+// This test requires the Blocks runtime, which is (only?) available
+// on Darwin out-of-the-box.
+// REQUIRES: has-fblocks && darwin
 
 // FILE_DEPENDENCIES: %t.exe
 // RUN: %{build} -fblocks
 // RUN: %{run}
 
-#if __has_include(<Block.h>)
-
 #include <functional>
 #include <cstdlib>
 #include <cassert>
 
 #include "test_macros.h"
 #include "count_new.h"
 
 
int main(int, char**)
     // target_type
     {
         std::function<int(int)> f = ^(int x) { return x + 1; };
         assert(f.target_type() == typeid(int(^)(int)));
     }
 
     return 0;
 }
-
-#else
-
-int main() { }
-
-#endif