Why does this allocator not work with `std::allocate_shared`? Bizarre template substitution errors

Demo

foo_allocator is a working allocator for stl containers. It wraps over a base allocator type and forwards allocate(), deallocate(), operator==, operator!=, etc to the base.

#include <iostream> #include <string> #include <vector> #include <memory> template <typename T> class bar_allocator : public std::allocator<T> {}; template <typename T, typename base=bar_allocator<T>> class foo_allocator { public: typedef typename std::allocator_traits<base>::value_type value_type; template <typename U, typename A> friend class foo_allocator; template<class U> struct rebind { typedef foo_allocator<U, typename std::allocator_traits<base>::template rebind_alloc<U>> other; }; // Construct a dummy allocator from another dummy allocator with the same base_allocator but with different type. template <typename U> foo_allocator( const foo_allocator<U, typename std::allocator_traits<base>::template rebind_alloc<U>>& other) noexcept : alloc(other.alloc) {} foo_allocator() = default; template<typename... Args> foo_allocator(Args &&... args) requires (std::is_constructible_v<base, Args...>) : alloc(std::forward<Args>(args)...) {} T* allocate(std::size_t n) { T* p = alloc.allocate(n); return p; } void deallocate(T* p, std::size_t size) noexcept { alloc.deallocate(p, size); } private: base alloc; }; template <typename T, typename U, typename base_allocator> inline bool operator == (const foo_allocator<T, base_allocator>& a, const foo_allocator<U, typename std::allocator_traits<base_allocator>::template rebind_alloc<U>>& b) { return a.alloc == b.alloc; } template <typename T, typename U, typename base_allocator> inline bool operator != (const foo_allocator<T, base_allocator>& a, const foo_allocator<U, typename std::allocator_traits<base_allocator>::template rebind_alloc<U>>& b) { return a.alloc != b.alloc; } int main() { // Works fine std::vector<int, foo_allocator<int>> v; for (int i = 0; i < 100; i++) v.push_back(i); // Breaks! // foo_allocator<int> foo; // std::shared_ptr<int> ptr2 = std::allocate_shared<int>(foo); } 

However, it doesn't work with allocate_shared. If you try to allocate_shared with this allocator, you get a compiler error of the following:

In file included from /usr/local/include/c++/9.2.0/bits/shared_ptr.h:52, from /usr/local/include/c++/9.2.0/memory:81, from main.cpp:4: /usr/local/include/c++/9.2.0/bits/shared_ptr_base.h: In instantiation of 'std::__shared_count<_Lp>::__shared_count(_Tp*&, std::_Sp_alloc_shared_tag<_Alloc>, _Args&& ...) [with _Tp = int; _Alloc = foo_allocator<int>; _Args = {}; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]': /usr/local/include/c++/9.2.0/bits/shared_ptr_base.h:1344:71: required from 'std::__shared_ptr<_Tp, _Lp>::__shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = foo_allocator<int>; _Args = {}; _Tp = int; __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]' /usr/local/include/c++/9.2.0/bits/shared_ptr.h:359:59: required from 'std::shared_ptr<_Tp>::shared_ptr(std::_Sp_alloc_shared_tag<_Tp>, _Args&& ...) [with _Alloc = foo_allocator<int>; _Args = {}; _Tp = int]' /usr/local/include/c++/9.2.0/bits/shared_ptr.h:701:14: required from 'std::shared_ptr<_Tp> std::allocate_shared(const _Alloc&, _Args&& ...) [with _Tp = int; _Alloc = foo_allocator<int>; _Args = {}]' main.cpp:71:62: required from here /usr/local/include/c++/9.2.0/bits/shared_ptr_base.h:676:43: error: no matching function for call to 'foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >::foo_allocator(const foo_allocator<int>&)' 676 | typename _Sp_cp_type::__allocator_type __a2(__a._M_a); | ^~~~ main.cpp:32:5: note: candidate: 'foo_allocator<T, base>::foo_allocator(Args&& ...) requires is_constructible_v<base, Args ...> [with Args = {const foo_allocator<int, bar_allocator<int> >&}; T = std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>; base = std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> >]' 32 | foo_allocator(Args &&... args) requires (std::is_constructible_v<base, Args...>) : alloc(std::forward<Args>(args)...) {} | ^~~~~~~~~~~~~ main.cpp:32:5: note: constraints not satisfied main.cpp:32:5: note: 'is_constructible_v<base, Args ...>' evaluated to false main.cpp:29:5: note: candidate: 'constexpr foo_allocator<T, base>::foo_allocator() [with T = std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>; base = std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> >]' 29 | foo_allocator() = default; | ^~~~~~~~~~~~~ main.cpp:29:5: note: candidate expects 0 arguments, 1 provided main.cpp:24:5: note: candidate: 'template<class U> foo_allocator<T, base>::foo_allocator(const foo_allocator<U, typename std::allocator_traits<_Alloc>::rebind_alloc<U> >&)' 24 | foo_allocator( | ^~~~~~~~~~~~~ main.cpp:24:5: note: template argument deduction/substitution failed: In file included from /usr/local/include/c++/9.2.0/bits/shared_ptr.h:52, from /usr/local/include/c++/9.2.0/memory:81, from main.cpp:4: /usr/local/include/c++/9.2.0/bits/shared_ptr_base.h:676:43: note: mismatched types 'std::allocator<_CharT>' and 'bar_allocator<int>' 676 | typename _Sp_cp_type::__allocator_type __a2(__a._M_a); | ^~~~ main.cpp:10:7: note: candidate: 'constexpr foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >::foo_allocator(const foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >&)' 10 | class foo_allocator { | ^~~~~~~~~~~~~ main.cpp:10:7: note: no known conversion for argument 1 from 'const foo_allocator<int>' to 'const foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >&' main.cpp:10:7: note: candidate: 'constexpr foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >::foo_allocator(foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >&&)' main.cpp:10:7: note: no known conversion for argument 1 from 'const foo_allocator<int>' to 'foo_allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator<std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > >&&' 

What I can gather from the error messages

1. std::shared_ptr does some rebinding to create a struct that wraps over the int we're trying to allocate. In particular, it tries to allocate the type std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, which I'm guessing has the int but also other fields used to make sure shared_ptrs are threadsafe.
2. As a result, we have this really ugly struct that does some self-referencing with foo_allocator:

foo_allocator< std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic>, std::allocator< std::_Sp_counted_ptr_inplace<int, foo_allocator<int>, __gnu_cxx::_S_atomic> > > 

But wait! Why is std::allocator<> here? We never specified std::allocator<> as our base allocator! It should've been bar_allocator!

1. Later down the error log, we see the following bizarre line:

mismatched types 'std::allocator<_CharT>' and 'bar_allocator<int>' 

Other than the std::allocator<> showing up again, where did _CharT come from? doesn't this template type usually show up in strings?

Any help would be greatly appreciated. I've scratched my head about this for a while and couldn't come up with any reasonable fixes.