我如何理解“实例化点”的含义

Question

I am now learning some meta-programming skills from non-constant-constant-expressions , and I feel confused about "Point of Instantiation". The author says

Whenever a template specialization is referenced in a context that requires instantiation, that context gives birth to a "point of instantiation" (which effectively denotes a location where the compiler is allowed to generate code for the referenced template specialization).

If a template specialization X is referenced in a context that depends on a template-parameter of some surrounding template Y, the given point of instantation depends on the point of instantation of Y.

• If X is a function template specialization, the point of instantiation is that of Y.
• If X is a class template specialization, the point of instantiation is immediately before the point of instantiation of Y.

Otherwise, the given point of instantiation is tied to the location of the namespace scope declaration/definition (D) which contains the statement referring to X.

• If X is a function template specialization, the point of instantiation is immediately after D.
• If X is a class template specialization, the point of instantiation is immediately before D.

I don't know what is "a template specialization X is referenced in a context that depends on a template-parameter of some surrounding template Y", does he mean

template<typename T>
class Y{
    template<typename U1> void X1(){...};
    template<typename U2> class X2{...};
}

Consider Y<int>::X1<int>(...) , X1 and Y instantiate at the same time.

Consider Y<int>::X2<int>{...} , X2 first Y second.

Answer 1

It just means that, given

template<class> class A {};
template<class T> void f(T) {}
template<class T> void g() {f(A<T>{});}

the point of instantiation of (say) A<int> is immediately before that shared by f<int> and g<int> . (Remember that function (but not class) templates can have more than one point of instantiation in a translation unit.)