Is instantiating a class template with an incomplete type ill-formed, if the type is defined afterwards?

Question

This code is surely ill-formed, because Foo is specialized after an instantiation point:

template 
struct Foo {
    int a;
};

Foo         


        

Answer 1

Fortunatly, yes this is well-defined. For the exact same reason this is well-defined:

struct A;

class Foo { A* value; };

Foo foo; // note A is incomplete here

struct A {};

and this is ill-formed:

struct A;

template  class Foo { T value; }; // error: 'Foo::value' has incomplete type

Foo foo; // note A is incomplete here

struct A {};