nullptr in C++ mode on macOS

[Bruno Haible]

On macOS 12.5 (machine: gcc104.fsffrance.org), the test-nullptr-c++.cc fails
to compile:

 -----------------------------------------------------------------------------
Making check in .
c++ -DHAVE_CONFIG_H -I. -I../../gltests -I..  -DGNULIB_STRICT_CHECKING=1 
-DIN_GNULIB_TESTS=1 -I. -I../../gltests -I.. -I../../gltests/.. -I../gllib 
-I../../gltests/../gllib -I/Users/haible/include -Wall  -Wno-error -g -O2 -MT 
test-nullptr-c++.o -MD -MP -MF .deps/test-nullptr-c++.Tpo -c -o 
test-nullptr-c++.o ../../gltests/test-nullptr-c++.cc
../../gltests/test-nullptr-c++.cc:56:27: error: cannot pass object of non-POD 
type 'std::nullptr_t' through variadic function; call will abort at runtime 
[-Wnon-pod-varargs]
  varargs_callee ("type", nullptr, "foo", nullptr);
                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__nullptr:48:17:
 note: expanded from macro 'nullptr'
#define nullptr _VSTD::__get_nullptr_t()
                ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:858:15:
 note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_ABI_NAMESPACE
              ^
../../gltests/test-nullptr-c++.cc:56:43: error: cannot pass object of non-POD 
type 'std::nullptr_t' through variadic function; call will abort at runtime 
[-Wnon-pod-varargs]
  varargs_callee ("type", nullptr, "foo", nullptr);
                                          ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__nullptr:48:17:
 note: expanded from macro 'nullptr'
#define nullptr _VSTD::__get_nullptr_t()
                ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/__config:858:15:
 note: expanded from macro '_VSTD'
#define _VSTD std::_LIBCPP_ABI_NAMESPACE
              ^
2 errors generated.
make[3]: *** [test-nullptr-c++.o] Error 1
 -----------------------------------------------------------------------------

What's happening? configure found out that

  checking for C nullptr... no
  checking for C++ nullptr... no

and accordingly, config.h contains

/* #undef HAVE_CXX_NULLPTR */
/* #undef HAVE_C_NULLPTR */

Here, nullptr is not defined as a keyword (as it should, cf.
https://en.cppreference.com/w/cpp/keyword/nullptr), but rather as a macro,
and only after some header file such as <stddef.h> has been included.

Our test "checking for C++ nullptr... no" found it out correctly.

But test-nullptr-c++.cc includes <iostream>, <vector>, etc. and these have
the effect of pulling a definition of nullptr_t and

  #define nullptr _VSTD::__get_nullptr_t()

and this one does not work in varargs context.

So, we've now gone a full circle:
  * nullptr and nullptr_t were introduced in C++, to resolve a problem with
    overloading and a problem with varargs.
  * The definition in this compiler solves the problem with overloading,
    but is unusable with varargs.
  * If we '#define nullptr __null' again after including the header files,
    it will be usable with varargs, but will
      - violate the requirement that nullptr is an instance of nullptr_t,
      - and thus open up problems with overloading again.

I don't know how to proceed here. C++ is such a waste of time!!

Bruno