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template <class T>
struct Foo
{
 
    template <class U>
    struct Bar
    {
    	Bar();
 
        template <class X, class Y>
        void foo(typename Foo<X>::template Bar<Y> b)
        {
 
        }
    };
 
};
 
int main()
{
	//Foo<int>::Bar<int> b = Foo<char>::Bar<char>();
	Foo<int>::Bar<int> i;
	i.foo(Foo<char>::Bar<char>());
}

prog.cpp: In function ‘int main()’:
prog.cpp:23:30: error: no matching function for call to ‘Foo<int>::Bar<int>::foo(Foo<char>::Bar<char>)’
  i.foo(Foo<char>::Bar<char>());
                              ^
prog.cpp:23:30: note: candidate is:
prog.cpp:11:14: note: template<class X, class Y> void Foo<T>::Bar<U>::foo(const typename Foo<X>::Bar<Y>&) [with X = X; Y = Y; U = int; T = int]
         void foo(const typename Foo<X>::template Bar<Y>& b)
              ^
prog.cpp:11:14: note:   template argument deduction/substitution failed:
prog.cpp:23:30: note:   couldn't deduce template parameter ‘X’
  i.foo(Foo<char>::Bar<char>());
                              ^

Standard output is empty