c++ - gcc doesn't accept pack expansion in default template argument -

following code compiled clang, gcc fails:

struct fn {     template <typename ... args>         static constexpr bool call (args ... ) { return true; } };  template <typename ... t>     static constexpr bool f = false;  template <typename ... ts, bool f = fn::call(f<ts> ...)>     void hoge () {}  int main () {} 

gcc 5.1.0 (-wall -wextra -std=c++14 -pedantic) says

prog.cc:10:52: error: expansion pattern 'f<ts>' contains no argument packs template <typename ... ts, bool f = fn::call(f<ts> ...)> 

clang 3.6.0 , 3.5.0 gives no errors.

am , clang violating c++ rules or gcc bug?

you haven't violated rule. appears problem gcc's support variable templates, not default arguments, because adjustment works:

template <typename ... t> struct f {     static constexpr bool v = false; };  template <typename ... ts, bool f = fn::call(f<ts>::v ...)>     void hoge () {} 

http://coliru.stacked-crooked.com/a/ff81b6ab052a748b

as far know, variable template equivalent class template wrapping static member, shouldn't cause problems besides needing write ::v.