[Bug c++/107022] New: error: use of deleted function 'std::unordered_map

[Bug c++/107022] New: error: use of deleted function 'std::unordered_map

[piersh at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

            Bug ID: 107022
           Summary: error: use of deleted function 'std::unordered_map
           Product: gcc
           Version: 11.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: piersh at hotmail dot com
  Target Milestone: ---

this is a regression in newer point-releases of gcc (this bug got backported).
it appears first in 10.4, 11.2.1 and 12.x. 10.3 and 11.2.0 do not have this
issue.


#include <unordered_map>
#include <memory>

struct key {};

struct foo
{
    struct my_hash
    {
        my_hash(int i = 42) {}
        std::size_t operator()(const key &ep) const { return 0; }
    };

private:
    std::unordered_map<key, std::weak_ptr<std::string>, foo::my_hash>
things_{};
};


<source>:15:79: error: use of deleted function 'std::unordered_map<_Key, _Tp,
_Hash, _Pred, _Alloc>::unordered_map() [with _Key = key; _Tp =
std::weak_ptr<std::__cxx11::basic_string<char> >; _Hash = foo::my_hash; _Pred =
std::equal_to<key>; _Alloc = std::allocator<std::pair<const key,
std::weak_ptr<std::__cxx11::basic_string<char> > > >]'
   15 |     std::unordered_map<key, std::weak_ptr<std::string>, foo::my_hash>
things_{};
      |                                                                        
      ^
In file included from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/unordered_map:47,
                 from <source>:1:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/unordered_map.h:141:7:
note: 'std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::unordered_map()
[with _Key = key; _Tp = std::weak_ptr<std::__cxx11::basic_string<char> >; _Hash
= foo::my_hash; _Pred = std::equal_to<key>; _Alloc =
std::allocator<std::pair<const key,
std::weak_ptr<std::__cxx11::basic_string<char> > > >]' is implicitly deleted
because the default definition would be ill-formed:
  141 |       unordered_map() = default;
      |       ^~~~~~~~~~~~~
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/unordered_map.h:141:7:
error: use of deleted function 'std::_Hashtable<_Key, _Value, _Alloc,
_ExtractKey, _Equal, _Hash, _RangeHash, _Unused, _RehashPolicy,
_Traits>::_Hashtable() [with _Key = key; _Value = std::pair<const key,
std::weak_ptr<std::__cxx11::basic_string<char> > >; _Alloc =
std::allocator<std::pair<const key,
std::weak_ptr<std::__cxx11::basic_string<char> > > >; _ExtractKey =
std::__detail::_Select1st; _Equal = std::equal_to<key>; _Hash = foo::my_hash;
_RangeHash = std::__detail::_Mod_range_hashing; _Unused =
std::__detail::_Default_ranged_hash; _RehashPolicy =
std::__detail::_Prime_rehash_policy; _Traits =
std::__detail::_Hashtable_traits<true, false, true>]'
In file included from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/unordered_map:46:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/hashtable.h:529:7:
note: 'std::_Hashtable<_Key, _Value, _Alloc, _ExtractKey, _Equal, _Hash,
_RangeHash, _Unused, _RehashPolicy, _Traits>::_Hashtable() [with _Key = key;
_Value = std::pair<const key, std::weak_ptr<std::__cxx11::basic_string<char> >
>; _Alloc = std::allocator<std::pair<const key,
std::weak_ptr<std::__cxx11::basic_string<char> > > >; _ExtractKey =
std::__detail::_Select1st; _Equal = std::equal_to<key>; _Hash = foo::my_hash;
_RangeHash = std::__detail::_Mod_range_hashing; _Unused =
std::__detail::_Default_ranged_hash; _RehashPolicy =
std::__detail::_Prime_rehash_policy; _Traits =
std::__detail::_Hashtable_traits<true, false, true>]' is implicitly deleted
because the default definition would be ill-formed:
  529 |       _Hashtable() = default;
      |       ^~~~~~~~~~
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/hashtable.h:529:7:
error: use of deleted function 'constexpr
std::_Enable_default_constructor<false, _Tag>::_Enable_default_constructor()
[with _Tag = std::__detail::_Hash_node_base]'
In file included from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/hashtable.h:36:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/enable_special_members.h:113:15:
note: declared here
  113 |     constexpr _Enable_default_constructor() noexcept = delete;
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiler returned: 1

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is another when is the inner type complete and the constructor can be used
issues really.

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Mainly read bug 102199 comment #7

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, and the standard says that instantiating std::unordered_map with
incomplete types is undefined.

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[piersh at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #4 from piersh at hotmail dot com ---
is it related to nested classes? this reproduces the issue: uncomment line 9 to
repro:


#include <unordered_map>
#include <memory>

struct key {};

struct my_hash
{
    my_hash() {}
    //my_hash(int i = 42) {}  // <<-- uncomment for bug
    std::size_t operator()(const key &ep) const { return 0; }
};

struct foo
{
    foo();
private:
    std::unordered_map<key, int, my_hash> things_;
};

foo::foo() {}

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[piersh at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #5 from piersh at hotmail dot com ---
oh, no. scratch that last comment.

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to piersh from comment #4)
> struct my_hash
> {
>     my_hash() {}
>     my_hash(int i = 42) {}  // <<-- uncomment for bug
>     std::size_t operator()(const key &ep) const { return 0; }
> };

That is because my_hash constructor call with no arguments is ambigous at that
point. If you define one or the other constructor, it is no longer ambigous. If
you define both, the code is invalid.

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jens.maurer at gmx dot net

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 107340 has been marked as a duplicate of this bug. ***

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[jens.maurer at gmx dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #8 from Jens Maurer <jens.maurer at gmx dot net> ---
I understand we're in a tangled web of partially-complete class rules here,
but the standard does give you the expectation that the inner class is complete
at its closing brace:

[class.pre] p2 says:

"A class is considered defined after the closing brace of its class-specifier
has been seen even though its member functions are in general not yet defined."

[Bug c++/107022] error: use of deleted function 'std::unordered_map

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107022

--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> ---
For the example in comment 0 the default-initializer for things_{} is the
problem, removing the {} allows it to compile.

For the example in Bug 107340 the problem is the default-initializer x = 0 in
the nested class, removing that allows it to compile.

The nested class is complete at the closing } but its complete class contexts
are not complete until the enclosing class is complete. default-initializers
and nested classes do not work well.