[cfe-dev] std::enable_if<T::a> with enum T::a = 2

[Richard Smith]

On Thu, Sep 13, 2012 at 9:21 AM, Richard Smith <richard at metafoo.co.uk>wrote:

> Hello,
>
> On Thu, Sep 13, 2012 at 7:31 AM, Ansgar Burchardt <
> ansgar.burchardt at iwr.uni-heidelberg.de> wrote:
>
>> Hi,
>>
>> I'm not sure if this is the right list, but I encountered a problem with
>> std::enable_if in clang++:
>>
>> ---
>> #include <iostream>
>> #include <type_traits>
>>
>> template<int n>
>> struct A {
>>   enum { a = n };
>> };
>>
>> template<class T, typename dummy = void>
>> struct B
>> {
>>   enum { b = 1 };
>> };
>>
>> template<class T>
>> struct B<T, typename std::enable_if<T::a>::type>
>> {
>>   enum { b = 2 };
>> };
>>
>> int main(void)
>> {
>>   std::cout << B< A<1> >::b << std::endl;
>>   std::cout << B< A<2> >::b << std::endl;
>> }
>> ---
>>
>> This will give "2 1" with clang 3.1-8 (Debian), but "2 2" with g++
>> 4.7.1-8 (Debian).  I'm far from being an expert in C++, but shouldn't
>> both lines use the specialized version of B (ie. g++ is correct)?
>>
>
> No, this program's behavior changed in C++11. Converting a constant 2 to
> bool is a narrowing conversion, which is disallowed in a converted constant
> expression such as a non-type template argument. G++ apparently doesn't
> implement that rule yet.
>
> The second result should also be '1' in C++11 mode -- this is another
> C++11-specific change. I brought this to the committee's attention in
> DR1407, and we agreed that this was the desired behavior, but no compiler
> has implemented it yet (as far as I'm aware).
>

I implemented DR1407 in r163829. We now produce "1 1" in C++11 mode and "2
2" in C++98 mode.
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