Reference variable and initializer lists

Question

I am using protobuf, and I was assigning a const reference to another reference, this seems to trigger the copy constructor if I use {}, but works fine if I use ().

For example,

// Protobuf Messages
message Test {
    string x = 1;
}

message List {
   repeated Test tests = 1;
}


int main()
{
    List l {};
    const google::protobuf::RepeatedPtrField<Test>& a = l.tests();

    const google::protobuf::RepeatedPtrField<Test>& b = a; // 1
    const google::protobuf::RepeatedPtrField<Test>& c {a}; // 2
    const google::protobuf::RepeatedPtrField<Test>& d (a); // 3
 }

Call (1, 3) works fine, but Call (2) seems to call the copy constructor defined as

template <typename Element>
inline RepeatedPtrField<Element>::RepeatedPtrField(
    const RepeatedPtrField& other)
  : RepeatedPtrFieldBase() {
  MergeFrom(other);
}

Could someone help me understand why Call (2) creates a copy, I seem to be misunderstanding a rule in the initializer list.

Thanks...