让我举例说明。让我们以std::is_same_v和std::is_base_of_v为例。请考虑以下代码：

#include <iostream>
#include <array>
#include <type_traits>

using namespace std;

struct base {};
struct derived : base { int foo; };

array<derived, 10> my_array;

int main()
{
    using c1 = decltype(*begin(my_array));
    using c2 = derived;

    if constexpr(is_same_v<c1,c2>)
    {
        cout<<"Correct!"<<endl;
    }
    else
    {
        cout << "No luck even though:" << endl
             << "c1 type is " << typeid(c1).name() << endl
             << "c2 type is " << typeid(c2).name() << endl;
    }

    if constexpr(is_base_of_v<base, c1>)
    {
        cout<<"Correct!"<<endl;
    }
    else
    {
        cout << "No luck even though:" << endl
             << "is_base_of_v<base, derived> = " << is_base_of_v<base, derived> << endl;
    }

    return 0;
}

预期的结果是：

Correct!
Correct!

但是，在clang 10和GCC11中的实际结果都是：

No luck even though:
c1 type is 7derived
c2 type is 7derived
No luck even though:
is_base_of_v<base, derived> = 1

精神错乱。我主要感兴趣的是找出这种行为背后的确切原因，然后可能找到一个变通办法。如果可能，该变通方法应该适用于任何可迭代类型，并且只适用于std：：array。