std::数组实现上的隐式构造函数出现奇怪错误

#include "array.hpp"
int main() {
constexpr array<int, 5> a1 = { { 2, 4, 6, 8 } };
}

这是我的代码^

我搜索了一下，据说隐式构造函数可以这样做{ { blah, blah, blah } }，但仍然不起作用。我也只是做了一个统一的大括号初始化语法(a1 = { 2, 4, 6, 8 }(，但没有成功。

错误：

main.cpp: In function ‘int main()’:
main.cpp:5:57: error: could not convert ‘{{2, 4, 6, 8}}’ from ‘<brace-enclosed initializer list>’ to ‘const xstl::array<int, 5>’
5 |     constexpr xstl::array<int, 5> a1 = { { 2, 4, 6, 8 } };
|                                                         ^
|                                                         |
|                                                         <brace-enclosed initializer list>
main.cpp:6:57: error: could not convert ‘{{1, 3, 5, 7}}’ from ‘<brace-enclosed initializer list>’ to ‘const xstl::array<int, 5>’
6 |     constexpr xstl::array<int, 5> a2 = { { 1, 3, 5, 7 } };
|                                                         ^
|                                                         |
|                                                         <brace-enclosed initializer list>

我不理解这个错误，因为构造函数是隐式的？

为什么这个聚合出错？为什么std::array实现有效，而此实现无效？

以下是实现，不太多：

#ifndef HEADER_ARRAY_HPP
#define HEADER_ARRAY_HPP
#include <cstddef>
#include <iterator>
#include <stdexcept>
template <class T, std::size_t N>
struct array {
using value_type = T;
using size_type = std::size_t;
using difference_type = std::ptrdiff_t;
using reference = value_type&;
using const_reference = const value_type&;
using pointer = value_type*;
using const_pointer = const value_type*;
using iterator = value_type*;
using const_iterator = const value_type*;
using reverse_iterator = std::reverse_iterator<iterator>;
using const_reverse_iterator = std::reverse_iterator<const_iterator>;
const_reference at(size_type pos) const {
return pos < size() ? Data[pos] : throw std::out_of_range("");
}
const_reference operator[](size_type pos) const {
return Data[pos];
}
reference front() {
return *begin();
}
const_reference front() const {
return *begin();
}
reference back() {
return *std::prev(end());
}
const_reference back() const {
return *std::prev(end());
}
T *data() noexcept {
return Data;
}
const T *data() const noexcept {
return Data;
}
iterator begin() noexcept {
return Data;
}
const_iterator begin() const noexcept {
return Data;
}
const_iterator cbegin() const noexcept {
return Data;
}
iterator end() noexcept {
return Data + size();
}
const_iterator end() const noexcept {
return Data + size();
}
const_iterator cend() const noexcept {
return Data + size();
}
reverse_iterator rbegin() noexcept {
return reverse_iterator(end());
}
const_reverse_iterator rbegin() const noexcept {
return const_reverse_iterator(end());
}
const_reverse_iterator crbegin() const noexcept {
return const_reverse_iterator(end());
}
reverse_iterator rend() noexcept {
return reverse_iterator(begin());
}
const_reverse_iterator rend() const noexcept {
return const_reverse_iterator(begin());
}
const_reverse_iterator crend() const noexcept {
return const_reverse_iterator(begin());
}
constexpr bool empty() const noexcept {
return begin() == end();
}
constexpr size_type size() const noexcept {
return N;
}
constexpr size_type max_size() const noexcept {
return N;
}
void fill(const T& value) {
size_type i = 0;
for (; i < size(); i++) {
Data[i] = value;
}
}
void swap(array& other) noexcept {
size_type i;
for (i = 0; i < other.size(); i++) {
other.Data[i] = Data[i];
}
for (i = 0; i < size(); i++) {
Data[i] = other.Data[i];
}
}
private:
T Data[N];
};
#endif // HEADER_ARRAY_HPP