[llvm-bugs] [Bug 39631] New: __has_feature(cxx_exceptions) returns true but fails to compile std::uncaught_exceptions()

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39631

            Bug ID: 39631
           Summary: __has_feature(cxx_exceptions) returns true but fails
                    to compile std::uncaught_exceptions()
           Product: clang
           Version: 6.0
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++'17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: noloader at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

I'm testing C+_17 on OS X 10.8.5 and 10.9.5 using Macports compilers versions
5.0 and 6.0. Attempting to compile the following program:

  $ cat test.cxx
  #if __EXCEPTIONS && __has_feature(cxx_exceptions)
  # include <exception>
  # define CXX17_EXCEPTIONS 1
  #endif

  void Foo()
  {
  #if defined(CXX17_EXCEPTIONS)
      if (std::uncaught_exceptions() == 0)
  #endif
      {
          int x = 0;
      }
  }

Results in:

$ /opt/local/bin/clang++-mp-5.0 -std=gnu++17 test.cxx -c
test.cxx:9:14: error: 'uncaught_exceptions' is unavailable: introduced in macOS
10.12
    if (std::uncaught_exceptions() == 0)
             ^
/opt/local/libexec/llvm-5.0/include/c++/v1/exception:130:63: note:
      'uncaught_exceptions' has been explicitly marked unavailable here
_LIBCPP_FUNC_VIS _LIBCPP_AVAILABILITY_UNCAUGHT_EXCEPTIONS int uncaught_e...
                                                              ^
1 error generated.

According to the LLVM 3.6 release notes
(https://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro),
I believe I am using the correct feature test:

    To reliably test if C++ exceptions are enabled,
    use __EXCEPTIONS && __has_feature(cxx_exceptions),
    else things won’t work in all versions of Clang in
    Objective-C++ files.

==========

$ /opt/local/bin/clang++-mp-6.0 --version
clang version 6.0.1 (tags/RELEASE_601/final)
Target: x86_64-apple-darwin13.4.0
Thread model: posix
InstalledDir: /opt/local/libexec/llvm-6.0/bin

$ /opt/local/bin/clang++-mp-5.0 --version
clang version 5.0.2 (tags/RELEASE_502/final)
Target: x86_64-apple-darwin13.4.0
Thread model: posix
InstalledDir: /opt/local/libexec/llvm-5.0/bin

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.9.5
BuildVersion:   13F1911

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