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

Dear Thomas and John,

On 30. Oct 2020, at 19:17, John Maddock via Boost wrote:

On 30/10/2020 15:39, Thomas Ferrand via Boost wrote:

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Le 30/10/2020 à 15:16, Hans Dembinski via Boost a écrit :

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On 30. Oct 2020, at 14:52, Hans Dembinski 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