Re: [boost] BOOST_STATIC_CONSTANT

Subject: Re: [boost] BOOST_STATIC_CONSTANT
From: Vladimir Batov (Vladimir.Batov_at_[hidden])
Date: 2017-08-17 01:57:16

On 2017-08-17 11:33, Gavin Lambert via Boost wrote:
> On 17/08/2017 12:45, Vladimir Batov wrote:
>> Thanks, Gavin. Is it how it is done these days? Nice. Incorporated. 
>> Many thanks... clang still fails... now I am starting to look 
>> suspiciously in clang's direction.
> 
> FYI, the complete program:
> 
> #include <type_traits>
> 
> template <typename T>
> class has_impl_ptr_type
> {
>     template <typename T1>
>     static typename T1::impl_ptr_type test(int);
>     template <typename>
>     static void test(...);
> public:
>     enum { value = !std::is_void<decltype(test<T>(0))>::value };
> };
> 
> struct A {...};
> 
> struct B
> {
>     typedef int impl_ptr_type;
> };
> 
> static_assert(!has_impl_ptr_type<A>::value, "");
> static_assert( has_impl_ptr_type<B>::value, "");
> static_assert( has_impl_ptr_type<B const>::value, "");
> 
> int main() { return 0; }
> 
> Compiles successfully for me with -std=c++11 on gcc 5.4.0 and clang
> 3.8.0, as well as VS2015.
> 
> So unless this is some kind of regression in clang-4, your problem
> probably lies elsewhere.

Indeed, when has_impl_ptr_type is a separate class, it works as you 
tested. What I initially had was that test was/is part of impl_ptr

template <typename T>
struct impl_ptr
{   ...
     enum { value = !std::is_void<decltype(test<T>(0))>::value };
}

... and T was declared as

struct T : impl_ptr<T>::shared { ... };

That is, it appears, when the compiler gets to look at impl_ptr<T>, it 
has T incomplete!.. and fails is such a weird way.

I think to reproduce that in your code, it'll have to be

template <typename T>
class has_impl_ptr_type
{ ...
     struct base;
};

template <typename T>
struct has_impl_ptr_type::base
{
     using  impl_ptr_type = base;
};

struct B : has_impl_ptr_type<B>::base
{ ...
};

static_assert( has_impl_ptr_type<B>::value, "");

I suspect he above to fail with clang.

So, I am untangling this knot as you described above. Many thanks.