incomplete types with std::map and std::variant

  • A+

Consider this simplified and very specific implementation of a recursive variant on top of std::variant:

#include <map> #include <variant>  struct recursive_tag;  template <typename...> struct RecursiveVariant;  template <> struct RecursiveVariant<int, std::map<int, recursive_tag>>     : std::variant<int, std::map<int, RecursiveVariant<int, std::map<int, recursive_tag>>>> {     using underlying = std::variant<int,           std::map<int, RecursiveVariant<int, std::map<int, recursive_tag>>>>;     using underlying::underlying; };   int main() {     RecursiveVariant<int, std::map<int, recursive_tag>> rv;  } 

This fails to compile on gcc 7/8 due to trying to instantiate std::pair<const int, recursive_tag>, which itself fails because recursive_tag is an incomplete type.

But, nothing in the compiler error call-stack indicates to me why std::pair<const int, recursive_tag> needs to be instantiated. The top line there is:

variant:252:48: required from ‘void std::__detail::__variant::__erased_dtor(_Variant&&) [with _Variant = const std::__detail::__variant::_Variant_storage<false, int, std::map<int, RecursiveVariant<int, std::map<int, recursive_tag, std::less<int>, std::allocator<std::pair<const int, recursive_tag> > > >, std::less<int>, std::allocator<std::pair<const int, RecursiveVariant<int, std::map<int, recursive_tag, std::less<int>, std::allocator<std::pair<const int, recursive_tag> > > > > > > >&; long unsigned int _Np = 0]

pointing to:

249   template<typename _Variant, size_t _Np> 250     void 251     __erased_dtor(_Variant&& __v) 252     { std::_Destroy(std::__addressof(__get<_Np>(__v))); } 

While type map<int, recursive_tag> is spelled in there, the actual map type that should be instantiated is map<int, RecursiveVariant<int, map<int, recursive_tag>>>... which should only necessitate the instantiation of pair<const int, RecursiveVariant<...>>.

Simply making recursive_tag complete (i.e. by adding {}) fixes the problem. But what causes the problem to begin with?


The line at issue calls


The need to perform ADL for __get is sufficient to trigger instantiation of any and all associated classes of the type of __v, i.e., _Variant, to look for potential friend functions (and function templates) with that name defined within these classes. That includes the pair that tripped you up.


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