Why I can use private methods from template functions

Question

I have template class MyClass

template <class T>
class MyClass
{
public:
  MyClass() { }
private:
  void PrivateFunction() { 
    std::cout << "Message From Private Function" << std::endl; 
  }
};

Now when I am trying to use PrivateFunction() inside another function compiler reports error, but when I am trying the same inside template function compiler doesn't show any error.

1. Compiler doesn't report error.

   template <class T>
   void f()
   {
     MyClass<int> a;
     a.PrivateFunction();
   }
   

2. Compiler reports error.

   void f()
   {
     MyClass<int> a;
     a.PrivateFunction();
   }
   

Answer 1

Why I can use private methods from template functions?

You can't. §14.6 [temp.res]/p4:

If no valid specialization can be generated for a template, and that template is not instantiated, the template is ill-formed, no diagnostic required.

---

I am interested why I cant see error before calling the function

A compiler may defer the analysis until the function template is instantiated. Still, this is an implementation-defined behavior, and your code remains ill-formed (the continuation of §14.6/p4):

[ Note: If a template is instantiated, errors will be diagnosed according to the other rules in this Standard. Exactly when these errors are diagnosed is a quality of implementation issue. — end note ]

GCC demo (issues an error only on instantiation)
Clang demo (issues an error without instantiation)