GCC's behavior differs from Clang and Visual C++ in the following examples:

#include<type_traits>

template<typename T, typename Enabler = void>
struct test1 : std::false_type {};

template<typename T>
struct test1<T, typename T::typedef_test>  : std::true_type {};

template<typename T, typename Enabler = void>
struct test2 : std::false_type {};

template<typename T>
struct test2<T,
     typename std::enable_if<
         T::enum_test,
         void
     >::type>  : std::true_type {};

struct object
{
     template<typename T, typename Enabler>
     friend struct test1;

     template<typename T, typename Enabler>
     friend struct test2;

     private:
         enum { enum_test = true };
         typedef void typedef_test;
};

// test1
static_assert(test1<object>::value, "failed");

// test2
static_assert(test2<object>::value, "failed");

Results (compiler, test1, test2):
g++ (4.7.2), pass, fail
g++ (4.8.0), fail, fail
clang++ (3.3), pass, pass
Visual C++ 2012, pass, pass

Which is correct?  I'd appreciate direction to the relevant sections of
the standard if at all possible.


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Am 14.12.2012 18:20, schrieb maowfijwef@amfoijefa.ca:
> GCC's behavior differs from Clang and Visual C++ in the following examples:
>
> #include<type_traits>
>
> template<typename T, typename Enabler = void>
> struct test1 : std::false_type {};
>
> template<typename T>
> struct test1<T, typename T::typedef_test>  : std::true_type {};
>
> template<typename T, typename Enabler = void>
> struct test2 : std::false_type {};
>
> template<typename T>
> struct test2<T,
>       typename std::enable_if<
>           T::enum_test,
>           void
>       >::type>  : std::true_type {};
>
> struct object
> {
>       template<typename T, typename Enabler>
>       friend struct test1;
>
>       template<typename T, typename Enabler>
>       friend struct test2;
>
>       private:
>           enum { enum_test = true };
>           typedef void typedef_test;
> };
>
> // test1
> static_assert(test1<object>::value, "failed");
>
> // test2
> static_assert(test2<object>::value, "failed");
>
> Results (compiler, test1, test2):
> g++ (4.7.2), pass, fail
> g++ (4.8.0), fail, fail
> clang++ (3.3), pass, pass
> Visual C++ 2012, pass, pass
>
> Which is correct?  I'd appreciate direction to the relevant sections of
> the standard if at all possible.

With the resolution of the issue

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#580

the code should be well-formed. It looks to me as if gcc hasn't yet
applied the resolution. I suggest to open a compiler bug for this.

HTH & Greetings from Bremen,

Daniel Kr   gler




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