c++ - g++ Bug with Partial Template Specialization -

i writing tmp-heavy code g++ (version 4.8.1_1, macports) , clang++ (version 3.3, macports). while g++ rejects following code listing unbridled fury, clang++ compiles grace , splendor.

• which complier in right? (i suspect it's g++, want reassurance others before submitting bug report.)
• do have easy or elegant workarounds suggest? (i need use template aliases, switching on structs, causes g++ accept code, not option.)

here code listing, made just you.

template <class... ts> struct sequence;  template <int t> struct integer;  // definition of `extents` causes g++ issue compile-time error. template <int... ts> using extents = sequence<integer<ts>...>;  // however, definition works without problems. // template <int... ts> // struct extents;  template <int a, int b, class current> struct foo;  template <int a, int b, int... ts> struct foo<a, b, extents<ts...>> {     using type = int; };  template <int b, int... ts> struct foo<b, b, extents<ts...>> {     using type = int; };  int main() {     using t = foo<1, 1, extents<>>::type;     return 0; } 

here g++'s output:

er.cpp: in function 'int main()': er.cpp:39:41: error: ambiguous class template instantiation 'struct foo<1, 1, sequence<> >'   using t = typename foo<1, 1, extents<>>::type;                                          ^ er.cpp:26:8: error: candidates are: struct foo<a, b, sequence<integer<ts>...> >  struct foo<a, b, extents<ts...>>         ^ er.cpp:32:8: error:                 struct foo<b, b, sequence<integer<ts>...> >  struct foo<b, b, extents<ts...>>         ^ er.cpp:39:43: error: 'type' in 'struct foo<1, 1, sequence<> >' not name type   using t = typename foo<1, 1, extents<>>::type;                                            ^ 

here clang++'s output:

thanks help!

this seems g++ bug because foo<b, b, extents> more specialized foo<a, b, extents> (te latter can match former matches, not vice versa), compiler should choose specialization.

as noted yourself, changing extents template alias class template, solves problem.