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#include <iostream>
 
template <typename T>
class A
{
public:
	template <typename U>
	A() {
		std::cout << sizeof(T) << ' ' << sizeof(U) << std::endl;
	}
};
 
int main() {
	A<float> a(A<float>::A<int>());
	return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:14:30: error: cannot call constructor ‘A<float>::A’ directly [-fpermissive]
  A<float> a(A<float>::A<int>());
                              ^
prog.cpp:14:30: error:   for a function-style cast, remove the redundant ‘::A’ [-fpermissive]
prog.cpp:14:30: error: no matching function for call to ‘A<float>::A()’
prog.cpp:14:30: note: candidates are:
prog.cpp:8:2: note: template<class U> A<T>::A()
  A() {
  ^
prog.cpp:8:2: note:   template argument deduction/substitution failed:
prog.cpp:14:30: note:   couldn't deduce template parameter ‘U’
  A<float> a(A<float>::A<int>());
                              ^
prog.cpp:4:7: note: A<float>::A(const A<float>&)
 class A
       ^
prog.cpp:4:7: note:   candidate expects 1 argument, 0 provided

Standard output is empty