Re: [RFC] Deprecate "implicit int" for main() in C++

[Jason Merrill <jason@redhat.com>]

On Wed, Apr 25, 2018 at 10:40 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
> On 25/04/18 14:53 +0100, Andrew Haley wrote:
>>
>> On 04/25/2018 01:23 PM, Jonathan Wakely wrote:
>>
>>> We enabled -Wreturn-type by default in GCC 8, so code using the
>>> extension will get warnings even without -Wall now. Users might want
>>> to use -Werror=return-type to ensure they aren't bitten by the new
>>> optimizations that assume control never reaches the end of a
>>> non-void function.
>>
>>
>> But ISO C++ allows control to reach the end of main(), and
>> automagically returns 0.  I guess you didn't mean that, but your reply
>> was confusing.
>>
>> N4659, Section 6.6.1 Para 5:
>>
>> If control flows off the end of the compound-statement of main, the
>> effect is equivalent to a return with operand 0 (see also 18.3).
>
>
> Yes, I should have said "never reaches the end of a non-void function
> other than main".
>
> -Wreturn-type doesn't warn about flowing off the end of main, because
> it's well-defined. There's definitely scope for confusion, because
> -Wreturn-type gives the main function special treatment in two ways:
>
> * Unlike normal functions, flowing off the end of main doesn't warn,
>  because the standard defines what happens there.
>
> * Unlike normal functions, G++ allows omitting the return type of
>  main.
>  This is a non-standard extension to C++ (implicit int return types
>  are allowed by C89 but not by any version of C++).
>
> What I'm proposing for deprecation is the non-standard extension that
> allows:
>
> main() { return 0; }
>
> More concretely, deprecating it for a few releases would allow us to
> apply the attached patch at some point in the future, so that instead
> of:
>
> rt.c:1:6: warning: ISO C++ forbids declaration of ‘main’ with no type
> [-Wreturn-type]
> main() { return 0; }
>      ^
>
> We'd get:
>
> rt.c:1:6: error: ISO C++ forbids declaration of 'main' with no type
> [-fpermissive]
> main() { return 0; }
>      ^

I'm still not sure what the advantage is of changing the warning to an
error, but I suppose I wouldn't object either.

Jason