c – 成员模板变量专门化

class B
{   
    public:
        template <typename X>
            static X var;

        B() { std::cout << "Create B " << __PRETTY_FUNCTION__ << std::endl; }

        template <typename T>
        void Print() { std::cout << "Value is " << var<T> << std::endl; }
};

它必须在类范围之外声明所有静态成员：

以下编译并按预期工作：

template<typename T> T B::var=9; // makes only sense for int,float,double...

但是如何将这样的var专门化为以下非工作代码(使用gcc 6.1的错误消息)：

template <> double B::var<double>=1.123;

失败：

main.cpp:49:23: error: parse error in template argument list
 template <> double B::var<double>= 1.123;
                       ^~~~~~~~~~~~~~~~~~
main.cpp:49:23: error: template argument 1 is invalid
main.cpp:49:23: error: template-id 'var<<expression error> >' for 'B::var' does not match any template declaration
main.cpp:38:22: note: candidate is: template<class X> T B::var<T>
             static X var;

template <> double B::var=1.123;

失败了

template <> double B::var=1.123;
                       ^~~
main.cpp:38:22: note: does not match member template declaration here
             static X var;

这里的语法是什么？