build: (fixes #730) add compiler workaround for ostream& operator<<(ostream&, nullptr_t)

build-support/compiler-workarounds/ostream-operator-nullptr.h

@@ -0,0 +1,15 @@
+#ifndef OSTREAM_OPERATOR_NULLPTR_H
+#define OSTREAM_OPERATOR_NULLPTR_H
+
+#include <iostream>
+#include <cstddef>
+
+namespace std
+{
+inline std::ostream &operator<< (std::ostream &os, std::nullptr_t ptr)
+{
+  return os << "nullptr";   //whatever you want nullptr to show up as in the console
+}
+}
+
+#endif //OSTREAM_OPERATOR_NULLPTR_H

build-support/custom-modules/ns3-compiler-workarounds.cmake

@@ -0,0 +1,50 @@
+# Copyright (c) 2022 Universidade de Brasília
+#
+# This program is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License version 2 as published by the Free
+# Software Foundation;
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+#
+# You should have received a copy of the GNU General Public License along with
+# this program; if not, write to the Free Software Foundation, Inc., 59 Temple
+# Place, Suite 330, Boston, MA  02111-1307 USA
+#
+# Author: Gabriel Ferreira <gabrielcarvfer@gmail.com>
+
+include(CheckCXXSourceCompiles)
+
+# Clang 6, 7 and 8 shipped with incomplete C++17 features and do not handle
+# ostream& operator<<(ostream& os, nullptr_t ptr)
+# https://gitlab.com/nsnam/ns-3-dev/-/issues/730
+check_cxx_source_compiles(
+  "
+    #include <iostream>
+    #include <cstddef>
+    inline std::ostream& operator << (std::ostream& os, std::nullptr_t ptr)
+    {
+      return os << \"nullptr\"; //whatever you want nullptr to show up as in the console
+    }
+    int main()
+    {
+        std::ostream os(NULL);
+        os << std::nullptr_t();
+        return 0;
+    }
+    "
+  MISSING_OSTREAM_NULLPTR_OPERATOR
+)
+
+if(${MISSING_OSTREAM_NULLPTR_OPERATOR})
+  message(
+    ${HIGHLIGHTED_STATUS}
+    "Using compiler workaround: compiling in \"ostream& operator<<(ostream&, nullptr_t)\""
+  )
+  add_definitions(
+    -include
+    ${CMAKE_CURRENT_SOURCE_DIR}/build-support/compiler-workarounds/ostream-operator-nullptr.h
+  )
+endif()

build-support/macros-and-definitions.cmake

@@ -495,6 +495,15 @@ macro(process_options)
     )
   endif()
 
+  # Honor CMAKE_CXX_STANDARD in check_cxx_source_compiles
+  # https://cmake.org/cmake/help/latest/policy/CMP0067.html
+  cmake_policy(SET CMP0066 NEW)
+  cmake_policy(SET CMP0067 NEW)
+
+  # After setting the correct CXX version, we can proceed to check for compiler
+  # workarounds
+  include(build-support/custom-modules/ns3-compiler-workarounds.cmake)
+
   if(${NS3_DES_METRICS})
     add_definitions(-DENABLE_DES_METRICS)
   endif()