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

[Eli Friedman]

On Sun, Oct 23, 2011 at 11:34 AM, Howard Hinnant <hhinnant at apple.com> wrote:
> 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>()))
>        {};
>
> I.e. remove the common_type wrapper around decltype.  However this will have to be #ifdef'd as clang chokes on it:
>
> type_traits:1185:13: error: expected class name
>            decltype((__is_assignable_test(declval<_Tp>(), declval<_Arg>())))
>            ^
> 1 error generated.
>
> I actually don't know /why/ clang doesn't like this as [class.derived]/p1 appears to allow it (looks like a clang bug to me).

Please file a bug.

-Eli