c++ - template incomplete type used in nested specifier -

i've looked through several on-topic questions didn't help, sorry if duplicated.

why inner incomplete type there? circular dependence?

p.s. not_used placeholder class keep specialization partial

template <class a> struct outer {     template <class not_used, class enabled = void>     struct inner     { static void inner() { std::cout << "not enabled\n"; } };      template <class not_used>     struct inner<not_used, typename std::enable_if<std::is_same<int, a>::value>::type>     { static void inner() { std::cout << "enabled\n"; } }; };  template <class a> void call_inner(a& a) {     outer<a>::template inner<void>::inner(); // #1 }  int main() {     int intvar = 10;     double doublevar = 1;     call_inner(intvar); // ok     call_inner(doublevar); // error @ #1: incomplete type ‘outer<double>::inner<void>’ used in nested name specifier } 

update

if change second specialization (not_used->type, is_same<int, a> -> is_same<int, type>)

template<typename type> struct inner<type, typename std::enable_if<std::is_same<int, type>::value>::type> { static void inner() { std::cout << "enabled\n"; } }; 

and call_inner (note inner<void>->inner<t>)

template <class t> void call_inner(t& t) {     outer<t>::template inner<t>::inner(); } 

everything compiles. why that? apparently, dependence on outer template parameter somehow changes instantiation process?

the clang error message more explicit:

ess.cpp:11:52: error: no type named 'type' in       'std::__1::enable_if<false, void>'; 'enable_if' cannot used disable       declaration   ...inner<not_used, typename std::enable_if<std::is_same<int, a>::value>::type>                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~ ess.cpp:18:5: note: in instantiation of template class 'outer<double>' requested       here     outer<a>::template inner<void>::inner(); // #1     ^ ess.cpp:26:5: note: in instantiation of function template specialization       'call_inner<double>' requested here     call_inner(doublevar); // error @ #1: incomplete type <bf>outer<dou... 

as have enable_if restrict instanciation == int, cannot instantiate outer<double>::inner<void>.