c – 显式转换运算符模板的优先级和模糊性

#include <iostream>
#include <stdexcept>
#include <cmath>

using namespace std;

class A
{
        public:
        template<typename T> explicit operator T() const // 1
        {
                cout << "operator T" << endl;
                return T();
        }

        template<typename T> explicit operator const T&() const // 2
        {
                cout << "operator const T&" << endl;
                throw runtime_error("operator const T&");
        }

        template<typename T> explicit operator T&() // 3
        {
                cout << "operator T&" << endl;
                throw runtime_error("operator T&");
        }


};


int main(int,char**)
{
        try
        {
                const A& a = A();
                cout << abs(static_cast<double>(a) - 3.14) << endl;
        }
        catch (const runtime_error&)
        {

        }

        return 0;
}

我遇到的问题是为static_cast转换选择的运算符.对于海湾合作委员会来说,这是一种预期的(1)案例.输出是：

operator T
3.14

但Clang拒绝使用以下输出编译它：

@H_639_7@main.cpp:37:20: error: ambiguous conversion for static_cast from 'const A' to 'double' cout << std::abs(static_cast<double>(a) - 3.14) << endl; ^~~~~~~~~~~~~~~~~~~~~~ main.cpp:10:32: note: candidate function [with T = double] template<typename T> explicit operator T() const ^ main.cpp:16:32: note: candidate function [with T = double] template<typename T> explicit operator const T&() const ^ 1 error generated.

为什么Clang考虑转换(2),当它显然需要在转换序列中进行额外的构造函数调用时,而(1)不会？它是否正确(而GCC是错误的)这样做？