Recursive unordered_map - c++

Question

I have a tree structure that internally uses unordered map
#include <unordered_map>
struct Node {
std::unordered_map<int, Node> children;
};
int main() {
Node a;
}
It works just fine on Apple clang 11.0.3 and MSVC v19.24, but it fails to compile on clang 10.0.0 and gcc 10.1
While regular std::map works just fine on all compilers. I failed to find the reason for this discrepancy. Is there any way to use std::unordered_map as a value for itself? Or pointers is the only solution here?
Here's the compiler explorer link https://godbolt.org/z/6eYch9
Here's an error from gcc:
#3 with x86-64 gcc 10.1
In file included from /opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/unordered_map:43,
from <source>:1:
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/stl_pair.h:
In instantiation of 'struct std::pair<const int, Node>':
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/ext/aligned_buffer.h:91:28:
required from 'struct __gnu_cxx::__aligned_buffer<std::pair<const int,
Node> >'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable_policy.h:233:43:
required from 'struct
std::__detail::_Hash_node_value_base<std::pair<const int, Node> >'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable_policy.h:279:12:
required from 'struct std::__detail::_Hash_node<std::pair<const int,
Node>, false>'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable_policy.h:1973:13:
required from 'struct
std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const
int, Node>, false> > >'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/hashtable.h:173:11:
required from 'class std::_Hashtable<int, std::pair<const int, Node>,
std::allocator<std::pair<const int, Node> >,
std::__detail::_Select1st, std::equal_to<int>, std::hash<int>,
std::__detail::_Mod_range_hashing,
std::__detail::_Default_ranged_hash,
std::__detail::_Prime_rehash_policy,
std::__detail::_Hashtable_traits<false, false, true> >'
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/unordered_map.h:105:18:
required from 'class std::unordered_map<int, Node>'
<source>:4:39: required from here
/opt/compiler-explorer/gcc-10.1.0/include/c++/10.1.0/bits/stl_pair.h:218:11:
error: 'std::pair<_T1, _T2>::second' has incomplete type
218 | _T2 second; ///< The second member
| ^~~~~~
<source>:3:8: note: forward declaration of 'struct Node'
3 | struct Node {
| ^~~~
Compiler returned: 1

Answer 1

STL containers are not required to work with incomplete types. If you don't mind extra indirection, then the workaround is std::map<int, std::unique_ptr<Node>>

Answer 2

It's the same problem as doing e.g.
struct Node
{
Node child; // An instance of the full structure
};
You can't use a structure (or class) before it's fully defined, which it is at the closing }.
You can however define pointers to the structure, because then the compiler don't need the full structure definition, only know the name of the structure:
struct Node
{
Node* child; // Pointer to the structure
};
So to solve your problem, you need a map of pointers:
std::unordered_map<int, Node*> children;

Answer 3

Running into this while coding a Trie data structure I've noticed that in the latest of clang (v14) and GCC (v12) I don't get the error and my code operates as expected but anything below those versions (on godbolt.com) I get the same error. This is all using -std=c++14 so it's should not be related to the standard.