The main change of this PR is that sysconfig.get_config_var('CXX') becomes None instead of: 'g++' (for example on Linux).

I'm not sure about removing the CXX variable. It seems to be used by a few files:

Lib/_osx_support.py:18: 'BLDSHARED', 'LDSHARED', 'CC', 'CXX',
Lib/_osx_support.py:23: _COMPILER_CONFIG_VARS = ('BLDSHARED', 'LDSHARED', 'CC', 'CXX')
Lib/_osx_support.py:254: cv_split[0] = cc if cv != 'CXX' else cc + '++'

Lib/distutils/sysconfig.py:219: get_config_vars('CC', 'CXX', 'CFLAGS',
Lib/distutils/sysconfig.py:231: if 'CXX' in os.environ:
Lib/distutils/sysconfig.py:232: cxx = os.environ['CXX']

Modules/makesetup:243: *.cc)  obj=`basename $src .cc`.o; cc='$(CXX)';;
Modules/makesetup:244: *.c++) obj=`basename $src .c++`.o; cc='$(CXX)';;
Modules/makesetup:245: *.C)   obj=`basename $src .C`.o; cc='$(CXX)';;
Modules/makesetup:246: *.cxx) obj=`basename $src .cxx`.o; cc='$(CXX)';;
Modules/makesetup:247: *.cpp) obj=`basename $src .cpp`.o; cc='$(CXX)';;

Can distutils, setuptools and pip expect the CXX environment variable to be set? Or does sysconfig have to define a CXX configuration variable?

sysconfig.customize_compiler() uses CXX environment variable if it's set, or use the CXX sysconfig configuration variable.

The Makefile values are included in sysconfig data file and are available through sysconfig. Are you sure that no 3rd party project relies on the settings?

It seems like removing CXX is dangerous: it seems to be used, so it would be safer... to just keep it, just in case of doubt :-) MAINCC and LDCXXSHARED don't seem to be used outside Python itself.

Are you sure that no 3rd party project relies on the settings?

I ran a code search in the pypa GitHub organization:

• https://github.com/search?q=org%3Apypa+MAINCC : MAINCC is only mentioned in one issue, in a dump of sysconfig variables. It's not used in the code.
• https://github.com/search?q=org%3Apypa+LDCXXSHARED : LDCXXSHARED is only mentioned in one issue, in a dump of sysconfig variables. It's not used in the code.
• https://github.com/search?q=org%3Apypa+CXX : CXX is used in the code, but it seems like it's more about the CXX environment variable. But there is a variant of sysconfig.customize_compiler() which uses the sysconfig environment variable.

makesetup and final Makefile rely on CXX variable.

Yep, as I wrote.

makesetup:

			*.cc)  obj=`basename $src .cc`.o; cc='$(CXX)';;
			*.c++) obj=`basename $src .c++`.o; cc='$(CXX)';;
			*.C)   obj=`basename $src .C`.o; cc='$(CXX)';;
			*.cxx) obj=`basename $src .cxx`.o; cc='$(CXX)';;
			*.cpp) obj=`basename $src .cpp`.o; cc='$(CXX)';;

Python Git repository has no file with extensions: .cc, .c++, .C, .cxx.

There are a few files with .cpp extensions:

Lib/test/_testcppext.cpp
PC/pyshellext.cpp
PC/python_uwp.cpp
Tools/msi/bundle/bootstrap/PythonBootstrapperApplication.cpp
Tools/msi/bundle/bootstrap/pch.cpp
Tools/msi/bundle/bootstrap/pythonba.cpp

• Lib/test/_testcppext.cpp is built with setuptools in test_cppext
• PC/pyshellext.cpp and PC/python_uwp.cpp: I don't think that the Windows build system (VS) uses makesetup.
• Tools/msi/bundle/: I don't expect it to use makesetup.

I only see "CXX" at one line in the final Makefile: "CXX='$(CXX)'", does it copy the environment variable or the Makefile variable?

buildbottest: all
		-@if which pybuildbot.identify >/dev/null 2>&1; then \
			pybuildbot.identify "CC='$(CC)'" "CXX='$(CXX)'"; \
		fi
		$(TESTRUNNER) -j 1 -u all -W --slowest --fail-env-changed --timeout=$(TESTTIMEOUT) $(TESTOPTS)

I only see "CXX" at one line in the final Makefile: "CXX='$(CXX)'", does it copy the environment variable or the Makefile variable?

It copies the Makefile variable CXX to an environment variable.

BTW, what did you conclude here? The cxx stuff stays?

BTW, what did you conclude here?

I'm not brave. I abandon this PR and I wrote PR #95651 instead which only removes the MAINCC variables but keeps unused CXX and LDCXXSHARED variables.