read of non-constexpr variable 'a' is not allowed in a constant expression

[chlict]

源代码：

template<typename T>
struct A {
    T data;

    constexpr A(const T &data) : data(data) {}

    constexpr A(T &&data) : data(data) {}

    constexpr T getData() {
        return data;
    }
};

template <typename T>
struct B {
    T x;

    constexpr B(T x) : x(x) {}

    // constexpr B(const B &other) : x(other.x) {}
};

TEST(Suite1, Test3) {
    auto a = A(B(1_c));
    static_assert(a.data.x == 1_c);
    static_assert(a.getData().x == 1_c);
}

编译出错：

/Users/chenlong/software/TensorExamples/test/test.cpp:32:19: error: static_assert expression is not an integral constant expression
    static_assert(a.getData().x == 1_c);
                  ^~~~~~~~~~~~~~~~~~~~
/Users/chenlong/software/TensorExamples/src/includes/Test.hpp:12:16: note: read of non-constexpr variable 'a' is not allowed in a constant expression
        return data;
               ^
/Users/chenlong/software/TensorExamples/src/includes/Test.hpp:12:16: note: in call to 'B(a.data)'
/Users/chenlong/software/TensorExamples/test/test.cpp:32:21: note: in call to '&a->getData()'
    static_assert(a.getData().x == 1_c);
                    ^
/Users/chenlong/software/TensorExamples/test/test.cpp:30:10: note: declared here
    auto a = A(B(1_c));
         ^
1 error generated.

注意：

1. a.data可以用于static_assert而a.getData()不可以
2. struct B中打开注释的一行就能编译通过