[cfe-dev] [PATCH] Libc++ Windows fixes

[Howard Hinnant]

On Oct 24, 2011, at 5:11 AM, Ruben Van Boxem wrote:

> 2011/10/23 Howard Hinnant <hhinnant at apple.com>
> On Oct 23, 2011, at 1:51 PM, Ruben Van Boxem wrote:
> 
> > Op 23 okt. 2011 18:51 schreef "Per Lindén" <per.linden at lumai.se> het volgende:
> > >
> > > Not sure this is worth mentioning, but since error C2516 is defined as
> > > (http://msdn.microsoft.com/en-us/library/4d8f5kf3%28v=VS.100%29.aspx)
> > > "The class is derived from a type name defined by a typedef statement.",
> > > so for example naively changing the second variant of common_type in the
> > > test case you posted to connect.microsoft.com to
> > >
> > >  template <class _Tp>
> > >    struct common_type<_Tp, void, void>
> > >    {
> > >    public:
> > >        _Tp && type;
> > >    };
> > >
> > > makes the test case compile under VS2010 for me. (probably breaking any
> > > intended functionality completely) In any case I get "error C2371:
> > > 'size_t' : redefinition; different basic types" from the test case, so
> > > maybe reducing it a bit can improve upon the miniscule chance of it
> > > being fixed...? You have my vote anyway.
> >
> > I compiled for 64-bit, it's preprocessed source code, so that's the reason. Anyhow, you're probably right. I'll try to reduce it more when I find the time.
> >
> > Howard, is this (the start of) a viable workaround?
> 
> I don't really see how.  common_type<T>::type must be a type, not an object.  It seems strange that VS2010 is letting you derive from an object but not a type.
> 
> You might try this:
> 
> template <class _Tp, class _Arg, bool = is_void<_Tp>::value || is_void<_Arg>::value>
> struct __is_assignable_imp
>    : public decltype(__is_assignable_test(declval<_Tp>(), declval<_Arg>()))
>        {};
> 
> 
> This shows me this error, signaling that decltype is buggy as I suspected:
> > testcase.cpp(90) : error C2062: type '' unexpected
> > testcase.cpp(90) : see reference to class template instantiation 'std::__is_assignable_imp<_Tp,_Arg,__formal>' being compiled
> 
> Which only makes the problem worse... I googled this (although seemingly unrelated), suggesting adding a std::identity to accomodate this bug, and this, suggesting an intermediate typedef to work around the bug (although I don't see this being possible in this case...

We removed std::identity<T> late in the C++11 cycle because common_type<T> is the same thing and we were having backwards compatibility problems with extensions named identity.

Howard