我有这个代码：

#include <string>
#include <cstdint>

typedef unsigned long long u64;

struct A{
    A(int a) : m_a(a){}
    A(u64 a) : m_a(a){}
    A(double a) : m_a(a){}
    u64 m_a;
};

struct B{
    B(int b) : m_b(b){}
    B(uint64_t b) : m_b(b){}
    B(double b) : m_b(b){}
    uint64_t m_b;
};

int main(){
    
    // This works
    unsigned long long a = std::stoull("10000");
    
    // And this
    auto a2 = A(a);

    // And this
    auto a3 = A(std::stoull("10000"));
    

    // This works
    uint64_t b = std::stoull("10000");
    
    // And this
    auto b2 = B(b);

    // But not this?
    auto b3 = B(std::stoull("10000"));

    return 0;
}

Godbolt
为什么Clang和GCC抱怨最后一个例子？难道不应该和第三个例子一样吗？有人能解释一下这是怎么回事吗？C++有哪些特性/规则？