Re: [Boost-users] [MPL] HAS_XXX gives the wrong answer after "enable_if" on incomplete type in GCC 4.6/7

On 18/12/2012, Nathan Ridge wrote:

I'm not sure which is the case either, but notice that a simple workaround is to define f after traitor_1 is complete (you can still *declare* f where you do now).

Yes, you are correct; my actual code is a fair bit more complicated, but there is definitely a workaround. Moreover, to change this to a compile-time error, I'll change the definition of "traitor_1" to include a static assertion as follows. struct traitor_1 { typedef void *trait; static_assert(has_trait::value, "traitor_1::trait missing"); }; Without a static assertion failing, real code would compile with the wrong value for "has_trait<>". I think adding this to the definition will fix most of the otherwise "silent" bugs. I'm still interested in whether this is a defect, or just a limitation of C++, if anyone has an opinion... Most interesting to me is that using "enable_if" changes the final value of "has_trait<T>" by observing it "too early"...