Re: [boost] [config] gcc-4.4 and BOOST_NO_SFINAE_EXPR

Subject: Re: [boost] [config] gcc-4.4 and BOOST_NO_SFINAE_EXPR
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2012-09-24 19:51:26

On 09/24/2012 08:15 PM, Eric Niebler wrote:

> I confess to not understanding how the config tests work. When I run
> "bjam toolset=gcc-4.4 config_test" in libs/config/tests, the tests pass
> whether or not S_NO_SFINAE_EXPR is defined for gcc-4.4. I'm clearly
> doing something wrong.

Looks like the test is not good enough then.
(I was the one who wrote that particular test, but I don't quite 
understand the Boost.Config testing mechanism either)

> Anyway, here is a test for sfinae-expr that fails with gcc-4.4:
>
>      template<typename T> T declval();
>
>      struct not_incrementable {};
>
>      struct inc {
>          template<typename T>
>          auto operator()(T t) const -> decltype(t++)
>              { return t++; }
>      };
>
>      template<typename T>
>      struct wrap
>      {};
>
>      template<typename A>
>      int try_inc(wrap<decltype(declval<inc>()(declval<A>()))> *) {
>          return 0;
>      }
>
>      template<typename A>
>      not_incrementable try_inc(...) {
>          return not_incrementable();
>      }
>
>      struct X {};
>
>      int main() {
>          int x = try_inc<int>(0); // OK
>          not_incrementable y = try_inc<X>(0); // OK, not_incrementable
>      }
>

SFINAE for expressions also works in C++03, so it would be better if you 
could have a C++03 testcase.