问当尝试使用大括号调用模板基类构造函数时引发

EN

class A {
    int x;
public:
    A(int x = 0) : x{ x } {}; // works as expected.
};

class B : public A {
    int y;
public:
    B(int x = 1, int y = 1) : A(x), y{ y } {}; // old syntax works obviously.
};

template<typename T>
class A {
    T x;
public:
    A(T x = 0) : x{ x } {}; // works as expected.
};

template<typename T>
class B : public A<T> {
    T y;
public:
    // Compilation fails on the following line (vs2015).
    // Compiler has an issue with A<T>{ X }. Replacing {} with ()
    // works as expected. Shouldn't it work with {} as well?
    // B(T x = 1, T y = 1) : A<T>( x ), y{ y } {};  
    B(T x = 1, T y = 1) : A<T>{ x }, y{ y } {};  
};

Error   C2059   syntax error: ','
Error   C2334   unexpected token(s) preceding '{'; skipping apparent function body

template<typename T>
class C : public A<T> {
    using A_alias = A<T>;
    T z;
public:
    // Why does this workaround work while the expected syntax fails
    // to compile?
    C(T x = 2, T z = 2) : A_alias{ x }, z{ z } {};
};