Can template inheritance from its other specialization be a circular dependency or is it a clang error

Question

Consider the following code:

#include <iostream> #include <utility> #include <tuple> template <class TargetIndices, class SourceIndices> struct assign; template <size_t... TargetIs, size_t... SourceIs> struct assign<std::index_sequence<TargetIs...>, std::index_sequence<SourceIs...>>: assign<std::index_sequence<TargetIs>, std::index_sequence<SourceIs>>... { template <class TargetTuple, class SourceTuple> assign(TargetTuple &target, const SourceTuple &source): assign<std::index_sequence<TargetIs>, std::index_sequence<SourceIs>>(target, source)... { } }; template <size_t FirstTI, size_t FirstSI> struct assign<std::index_sequence<FirstTI>, std::index_sequence<FirstSI>> { template <class TargetTuple, class SourceTuple> assign(TargetTuple &target, const SourceTuple &source) { std::get<FirstTI>(target) = std::get<FirstSI>(source); } }; int main() { std::tuple<int, int, int> t1 = std::make_tuple(0,0,0), t2 = std::make_tuple(1,2,3); assign<std::index_sequence<0,2>, std::index_sequence<2,0>>(t1, t2); std::cout << std::get<0>(t1) << " " << std::get<1>(t1) << " " << std::get<2>(t1) << std::endl; } 

clang ++ claims that there is a circular dependency in inheritance. g ++ compiles it without any problems. Which compiler is right?

Answer 1

It looks like a clang error.

Minimum playback is as follows:

 template <int...> struct q {}; template <int... Is> using w = q<Is...>; template <class T> struct a; template <int F> struct a<w<F>> {}; template <int... T> struct a<w<T...>>: a<w<T>>... {}; // g++ OK, clang++ breaks 

The template alias is required here. When the source template is used in the last line instead of the template alias, it compiles:

 struct a<q<T...>>: a<q<T>>... {}; // g++, clang++ OK