[C++ Patch] PR 67065 ("Missing diagnostics for ill-formed program with main variable instead of function")

[Ville Voutilainen]

>submitter noticed that, in violation of [basic.start.main], we don't reject
>as ill-formed the declaration of a 'main' variable in the global namespace.
>Not a big deal IMHO, but the below simple check works well for me on 
>x86_64-linux.

Just fyi, gcc accepts

decltype(main) x;

decltype(sizeof(decltype(main)*)) x;

which are "uses" of main and also violate [basic.start.main]/3.