trying to understand (mis)initialization

gistfile1.txt

struct A{
	T x;	
	A(): x{default_T} {};
	A(T t): x{t} {};
};

// Produces two fully initialized A's,
// with a->x == default_T a2->x == my_T.
// Good, fine.
A* a = new A{};
A* a2 = new A{my_T};

// Compiler error: "cannot initialize a variable of type 'A *' with an rvalue of type 'T'".
// I assume direct intialization to a pointer *can't* work here because I'm creating a temporary obect,
// but I do not understand how that relates to the message.
// Question: What does the compiler do with this statement that leads to that error message?
A* a3{my_x};

// Compiles (at least with g++ -std=c++11), 
// but segfaults on any attempt to access a4->x
// Questions:
//   * Am I correct that the compiler treats this as a declaration and makes no attempt to initialize?
//   * For didactic purposes only, is there a way to write this erroneous "initialization" so that it complains at compile time?
A* a4{};