Handling of main() function for freestanding

[Jonathan Wakely <jwakely.gcc@gmail.com>]

As part of implementing a C++23 proposal [1] to massively increase the
scope of the freestanding C++ standard library some questions came up
about the special handling of main() that happens for hosted
environments.

As required by both C++ (all versions) and C (since C99), falling off
the end of the main() function is not undefined, the compiler is
required to insert an implicit 'return 0' [2][3]. However, this
special handling only applies to hosted environments. For freestanding
the return type or even the existence of main is
implementation-defined. As a result, GCC gives a -Wreturn-type warning
for this code with -ffreestanding, but not with -fhosted:

int main() { }

Arsen (CC'd) has been working on the libstdc++ changes for the
freestanding proposal, and several thousand libstdc++ tests were
failing when using -ffreestanding, because of the -Wreturn-type
warnings. He wrote a patch to the compiler [4] to add a new
-fspecial-main flag which defaults to on for -fhosted, but can be used
with -ffreestanding to do the implicit 'return 0' (and so disable the
-Wreturn-type warnings) for freestanding as well. This fixes the
libstdc++ test FAILs.

However, after discussing this briefly with Jason it occurred to us
that if the user declares an 'int main()' function, it's a pretty big
hint that they do want main() to return an int. And so having
undefined behaviour do to a missing return isn't really doing anybody
any favours. If you're compiling for freestanding and you *don't* want
to return a value from main(), then just declare it as void main()
instead. So now we're wondering if we need -fspecial-main at all, or
if int main() and int main(int, char**) should always be "special",
even for freestanding. So Arsen wrote a patch to do that too [5].

The argument against making 'int main()' imply 'special main' is that
in a freestanding environment, a function called 'int main()' might be
just a normal function, not the program's entry point. And in that
case, maybe you really do want -Wreturn-type warnings. I don't know
how realistic that is.

So the question is, should Arsen continue with his -fspecial-main
patch, and propose it along with the libstdc++ changes, or should gcc
change to always make 'int main()' "special" even for freestanding?
void main() and long main() and other signatures would still be
allowed for freestanding, and would not have the implicit 'return 0'.

I have no horse in this race, so if the maintainers of bare metal
ports think int main() should not be special for -ffreestanding, so be
it. I hope the first patch to add -fspecial-main would be acceptable
in that case, and libstdc++ will use it when testing with
-ffreestanding.

[1] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1642r11.html
[2] https://eel.is/c++draft/basic.start.main#5.sentence-2
[3] https://cigix.me/c17#5.1.2.2.3.p1
[4] https://github.com/ArsenArsen/gcc/commit/7e67edaced33e31a0dd4db4b3dd404c4a8daba59
[5] https://github.com/ArsenArsen/gcc/commit/c9bf2f9ed6161a38238e9c7f340d2c3bb04fe443