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template< class T >
   auto min( T a, T b ) -> decltype(a)
   {
      return a < b ? a : b;
   }
 
 
 
 
int main()
{
    struct A{};
    auto x = min( 2, 3 ) ;// success
 
    auto a = A{};
    auto b = A{};
    auto c = min(a,b);// here is error    
}

prog.cpp: In function ‘int main()’:
prog.cpp:13:10: warning: unused variable ‘x’ [-Wunused-variable]
     auto x = min( 2, 3 ) ;// success
          ^
prog.cpp:17:10: warning: variable ‘c’ set but not used [-Wunused-but-set-variable]
     auto c = min(a,b);// here is error    
          ^
prog.cpp: In instantiation of ‘decltype (a) min(T, T) [with T = main()::A; decltype (a) = main()::A]’:
prog.cpp:17:21:   required from here
prog.cpp:4:16: error: no match for ‘operator<’ (operand types are ‘main()::A’ and ‘main()::A’)
       return a < b ? a : b;
                ^
prog.cpp: In function ‘decltype (a) min(T, T) [with T = main()::A; decltype (a) = main()::A]’:
prog.cpp:5:4: warning: control reaches end of non-void function [-Wreturn-type]
    }
    ^

Standard output is empty