c – 为什么这个对成员函数的调用是模糊的？

class Base{
public:
    void func(double a) = delete;
    void func(int a) const {}
};

int main(){
    Base base;

    base.func(1);
    return 0;
}

当使用clang编译时,会产生以下错误：

clang++ --std=c++11 test.cpp 
test.cpp:22:7: error: call to member function 'func' is ambiguous
    base.func(1);

用g,会产生警告：

g++ -std=c++11 test.cpp 
test.cpp: In function ‘int main()’:
test.cpp:22:13: warning: ISO C++ says that these are ambiguous,even though the worst conversion for the first is better than the worst conversion for the second: base.func(1);

为什么这个代码不明确？