Re: [boost] [mpl] int_<INT_MIN> fails to compile under gcc 4.6

Subject: Re: [boost] [mpl] int_<INT_MIN> fails to compile under gcc 4.6
From: Stirling Westrup (swestrup_at_[hidden])
Date: 2011-03-06 09:07:37

On Sun, Mar 6, 2011 at 7:24 AM, Philipp Reh <sefi_at_[hidden]> wrote:
> Stirling Westrup wrote:
>
>> On Sun, Mar 6, 2011 at 5:50 AM, Philipp Reh <sefi_at_[hidden]> wrote:
>>> Stirling Westrup wrote:
>>>
>>>> I'm not actually sure if this is a gcc regression, or a bug in mpl,
>>>> but the following code, which compiles under gcc 4.5, doesn't do so
>>>> under gcc 4.6:
>>>>
>>>> #include <climits>
>>>> #include <boost/mpl/int.hpp>
>>>> typedef boost::mpl::int_<INT_MIN> Foo;
>>>> Foo f;
>>>>
>>>>
>>>> It produces the errors:
>>>>
>>>> /usr/local/include/boost/mpl/aux_/integral_wrapper.hpp: In
>>>> instantiation of ‘mpl_::int_<-0x00000000080000000>’:
>>>> test_test.cpp:8:27:   instantiated from here
>>>> /usr/local/include/boost/mpl/aux_/integral_wrapper.hpp:73:96: error:
>>>> ‘2147483647’ is not a valid template argument for type ‘int’ because
>>>> it is a non-constant expression
>>>> /usr/local/include/boost/mpl/aux_/integral_wrapper.hpp:73:96: error:
>>>> overflow in constant expression [-fpermissive]
>>>>
>>>>
>>>> I've already created a trak ticket for this (#5268), but was wondering
>>>> if anyone had any insights as to its cause and/or any workarounds. My
>>>> guess is that its related to gcc rewriting parts of its constant
>>>> folding code to support constexpr.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>> I can confirm that. INT_MIN isn't the only problem, it fails for a lot of
>>> other values as well.
>>> Can you please raise this issue on the gcc bugtracker because the release
>>> is very close.
>>>
>>
>> Okay, I have done so. Its bug #48003 on the GCC bugtracker.
>>
>>
>>
>
> Thanks. After looking further into this I realized that mpl tries to
> calculate INT_MIN-1 which, of course, is undefined. But I'm still not sure
> if this is a gcc bug or not because the typedef isn't used and I don't know
> if gcc should evaluate this.

An equally important questions is what should mpl do if someone DOES
ask for the prior of int_<INT_MIN>. Should it generate an error, throw
an exception, return INT_MIN, or wrap around to INT_MAX?

I can see arguments for most of these. Maybe it needs to be
policy-defined somehow?

-- 
Stirling Westrup
Programmer, Entrepreneur.
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