g++ 和 clang++ 与可变参数容器的不同行为

Question

In order to practice with C++11, I'm playing with variadic templates.

In particular, I'm playing with a sort of recursive variadic container class ( onion ) and a function that returns the number of template types ( func() ).

I came across a case that the clang++ (3.5.0) can't compile while the g++ (4.9.2) compiles and runs with no problems.

I have simplified it as follows

#include <iostream>

template <typename F, typename ... O>
struct onion;

template <typename F, typename S, typename ... O>
struct onion<F, S, O...>
 {
   F               first;
   onion<S, O...>  others;
 };

template <typename L>
struct onion<L>
 { L  last; };


template <typename ... Args>
std::size_t func (onion<Args...> const &)
 { return sizeof...(Args); }


int main()
 {
   auto o = onion<int, char const *, float> { 23, { "abcd", {34.0f}} };

   std::cout << func(o) << '\n';

   return 0;
 }

clang++ (3.5.0) gives me the following compiler error

test.cpp:28:17: error: no matching function for call to 'func'
   std::cout << func(o) << '\n';
                ^~~~
test.cpp:20:13: note: candidate template ignored: substitution
      failure [with Args = <>]: too few template arguments for class template
      'onion'
std::size_t func (onion<Args...> const &)
            ^     ~~~~~
1 error generated.

g++ (4.9.2) compiles without problem and, running, output 3 .

My question is: who's right?

clang++, giving an error, or g++, compiling?

I add that if I rewrite func() in this way

template <typename X, typename ... Args>
std::size_t func (onion<X, Args...> const &)
 { return 1U + sizeof...(Args); }

or if I redefine onion in this way

template <typename ... O>
struct onion;

both clang++ and g++ compile without errors.

Answer 1

This looks like a compiler bug in clang 3.5. If I run the code with the trunk version it compiles nicely.