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

Howard Hinnant hhinnant at apple.com
Sun Oct 23 16:05:35 PDT 2011

On Oct 23, 2011, at 6:13 PM, Eli Friedman wrote:

> 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.



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