If I have something like this:
int a = 5;
auto* p = &a;
decltype(*p) b = 1;
I'd guess that decltype(*p) is int and not int& or const int&.
But then what about dereferencing an iterator, e.g.
std::vector<int> a { 5 };
auto it = a.begin();
decltype(*it) b = 1;
Is decltype(*it) int? But isn't the type of *it int&?
I suspect that the answer is contained in item (4) here, but I'm still confused about xvalue and prvalue.
Edit: In the original code that confused me, I must have had something equivalent to
const int a = 5;
auto* p = &a;
decltype(*p) b = 1;
and not realized that a is const, so decltype(*p) was const int& and that's why the third line was fine.
Both *p and *it give you an int&, which is a lvaue reference to an int. Since you have an lvalue decltype(lvalue) gives you a T&.
That means in both code blocks b is an int& and the code wont compile as you cannot bind a prvalue (1 in this case) to a non-const lvalue reference