More C type errors by default for GCC 14

[Florian Weimer]

* Jason Merrill:

> As a compromise, it should be possible to error by default only in
> cases where the implicit int (including as a return value) is either
> the source or target of a non-value-preserving conversion, as those
> are very likely to be bugs.  That seems desirable for both camps.

Not sure, malloc returning 31-bit pointers goes a long way towards
maintaining int/pointer compatibility for 64-bit systems, too.  People
might actually have a working implicit-declared malloc on 64-bit today,
and still argue against the changes.

> A simpler change to catch this particular bug would be to make
> -Wint-conversion an error by default iff the sizes differ.

That doesn't address other bugs around implicitly-declared functions,
such as the bool/int ABI incompatibility, or the lack of type checking
for arguments.  I don't see those programmer mistakes because I think
it's easier to debug them on your own: swapped arguments are visible in
the source code, and GDB probably gives you enough data as well.  So I
suspect that people figure this out on their own.  They are still
wasting time on this, but probably somewhat less.

Thanks,
Florian