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Subject: Re: [boost] [type_traits] revisiting is_virtual_base_of
From: John Maddock (jz.maddock_at_[hidden])
Date: 2018-05-10 07:54:40

On 02/02/2018 20:16, Steven Watanabe via Boost wrote:
> On 02/02/2018 12:13 PM, John Maddock via Boost wrote:
>> There is a problem with is_virtual_base_of in that it doesn't work
>> especially well - most of the time it works OK, but can be made to fail
>> catastrophically in a few situations, see for example:
>> Now... it turns out that with C++14 and later we now have is_detected
>> which can test the validity of almost any expression... and I'm
>> wondering if there's a cunning way to press this into action to create a
>> more accurate is_virtual_base_of, but I'm having trouble coming up with
>> anything.  Any bright ideas? Note that an explicit static_cast can't be
>> tested for, though we can use is_constructible if required.
> Testing for static_cast<Derived*>(Base*) mostly
> works, except for private (or protected) virtual inheritance.

Thanks Steven, it turns out there was a PR I'd been sitting on that used
just this technique - works an absolute treat for gcc and msvc including
for private and protected inheritance, but fails to build in every case
for clang :(


Hopefully someone will spot a workaround for clang....

Thanks, John.

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