[copied from closed (out of date) issue https://bugs.python.org/issue25946]

Reproduced with Python 3.9.6.

./configure` both prints `checking for g++... no` and 

    WARNING:
    
      By default, distutils will build C++ extension modules with "g++".
      If this is not intended, then set CXX on the configure command line.

if `/usr/bin/g++` is present and executable which doesn't seem to be constructive because it's quite common that one wants to use `/usr/bin/g++` as CXX compiler if available. In case incompatibilities exists with other C++ compilers there should a check and more detailed error message.

Furthermore the error message doesn't explain if a part of distutils won't be build because the message sounds like the C++ extension is built, but does it work without a C++ compiler?

Specifying `CXX` environment variable or `--with-cxx-main=/usr/bin/g++` `configure` option works fine.