Why SFINAE gets messed up when changing the place of the class template specialization? Is this a C++ bug?

Question

Following code gives compiler error which is expected (Demo):

  1 template<bool> struct Range;
  2 
  3 template<int value, typename = Range<true> > struct Unique;
  4 template<int value> struct Unique<value, Range<(value > 1)> > { typedef char type[1]; };
  5 template<int value> struct Unique<value, Range<(value > 2)> > { typedef char type[2]; };
  6 
  7 Unique<3>::type o1;
  8 Unique<3>::type o2;

Now, if I swap line-5 and line-7. Then there is NO compiler error !! Demo.

  5 Unique<3>::type o1;

  7 template<int value> struct Unique<value, Range<(value > 2)> > { typedef char type[2]; };

For o1, it's understandable to have no error, because specialization for (value > 2) is not yet visible. But why there no error for o2 also, which sees 2 matching specializations !?
My guess is that, compiler should be choosing the Unique<3>::type with some arbitrary name when it encounters for the 1st time and then replacing Unique<3>::type everywhere with that name.

Is this a compilation bug or C++ bug or C++ "feature" ?

Answer 1

A template is instantiated the first time it is needed (in the translation unit), not each time.