Re: [histogram][type_traits] strange compile error in boost.histogram for gcc-7.1+ with -std=c++17; possible bug in gcc?

From: Hans Dembinski (hans.dembinski_at_[hidden])
Date: 2020-10-30 21:34:26

Dear Thomas and John,

> On 30. Oct 2020, at 19:17, John Maddock via Boost <boost_at_[hidden]> wrote:
> 
> On 30/10/2020 15:39, Thomas Ferrand via Boost wrote:
>> Le 30/10/2020 à 15:16, Hans Dembinski via Boost a écrit :
>>>> On 30. Oct 2020, at 14:52, Hans Dembinski <hans.dembinski_at_[hidden]> wrote:
>>>> 
>>>> Dear John and the other gurus,
>>>> 
>>>> a strange bug was reported for Boost.Histogram. It is a difficult one for which I would like to ask for help. To me it looks like a bug in gcc.
>>>> 
>>>> Here is the issue:
>>>> https://github.com/boostorg/histogram/issues/290
>>> Here is an absolute minimal example which does not include boost at all.
>>> 
>>> https://godbolt.org/z/hrcYao
>>> 
>>> clang is ok, msvc is ok, gcc fails.
> 
> A bit of messing about shows that:
> 
> * Returning T is OK.
> 
> * Returning std::size_t is OK
> 
> * Returning "unsigned int" is OK
> 
> * Returning "signed int" is OK
> 
> But:
> 
> * Returning "unsigned" fails.
> 
> * Returning "signed" fails.
> 
> So is changing the return type to "unsigned int" a viable option?
> 
> Weird, and does look like a bug.
> 
> HTH, John.
> 
>>> 
>>> 
>>> _______________________________________________
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>> 
>> Not, an expert but I would say it looks like a gcc bug.
>> 
>> As a workaround the function in the nested namespace can be declared with a trailing return type, this doesn't seem to trigger the bug: https://godbolt.org/z/fY979r

thank you for the two easy work arounds. I didn't consider playing with the return type. Good that I can implement the workaround on my end. I will report this bug to the gcc folks.

Thanks,
Hans