Question

Code:

#ifdef _MSC_VER
#    pragma warning( disable: 4480 )     // enum base as "nonstandard extension"
#endif
enum ShouldBeFine: char { hola };

enum Choice { a, b, c };

template< Choice c > struct Traits;

template<> struct Traits<a> { typedef char Type; };
template<> struct Traits<b> { typedef wchar_t Type; };
template<> struct Traits<c> { typedef long Type; };

template< Choice c >
struct Blah
{
    enum X: typename Traits<c>::Type {};
};

int main()
{} 

Only after a Herculean effort to file a bug report with Microsoft, did it occur to me that maybe Visual C++ is correct to refuse it, and g++, which compiles the above fine, is maybe wrong?

EDIT Details: the code fails to compile with Visual C++ 10.0 and with the preview of Visual C++ 11.0. Those compilers spit out some rambling error avalanche beginning with an alleged syntax error. The code compiles fine with MinGW g++ 4.4.1. Dani reports that it compiles fine with CLang. Unfortunately Comeau Online does not support this language feature, so it can’t be decided in the way we often did for C++98, just give the code to Comeau.

Answer 1

If I am reading the grammar correctly, you are correct in that this should compile. enum-base is : type-specifier-seq, and type-specifier-seq seems to include pretty much any type name you can think of, including typename Traits<c>::Type. And all three specializations result in Type being integral, which is also required of the enum-base. So That looks kosher to me.