In article <mailman.3878.1510925890.27995.bug-gnu-emacs@gnu.org> you wrote:

> GCC uses .mm or .M for Objective-C++,
> cf. https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Overall-Options.html.
> However, neither `auto-mode-alist' nor `cc-other-file-alist' contain
> these file extensions.

I don't think there's an Objective-C++ mode out there in the wild.  So
what existing mode should be used for *.mm and *.M?  Is that mode a good
enough fit to be able, in good conscience, to use it for Obj-C++?