https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106176
Bug ID: 106176
Summary: Compiler diagnostic doesn't show where it's coming
from in my code
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Consider the following program, carefully reduced from real code:
#include <map>
// template <class T> using which = std::map<int, T>;
template <class T> using which = T;
struct C
{
C();
~C() = default;
static C create();
struct M
{
M(M&&) = default;
};
which<M> member;
};
C C::create()
{
C c;
return c;
}
This, when compiled with gcc 12.1 -std=c++20 emits
(https://godbolt.org/z/YhfzWTjKq):
<source>: In static member function 'static C C::create()':
<source>:24:12: error: use of deleted function 'C::C(const C&)'
24 | return c;
| ^
<source>:6:8: note: 'C::C(const C&)' is implicitly deleted because the default
definition would be ill-formed:
6 | struct C
| ^
<source>:6:8: error: use of deleted function 'constexpr C::M::M(const C::M&)'
<source>:13:12: note: 'constexpr C::M::M(const C::M&)' is implicitly declared
as deleted because 'C::M' declares a move constructor or move assignment
operator
13 | struct M
| ^
Compiler returned: 1
This diagnostic (a) points to the offending line in my code, precisely, and (b)
explains what the problem is. That's pretty good. It could be better: the real
problem here is that C's explicitly default destructor inhibited the
compiler-generated move constructor, why is why the copy is a problem to begin
with.
If we change the definition of the member from M to std::map<int, M> (by
changing which declaration of which is commented), nothing really changes about
this program - we still have a move-only member, so this is broken since C is
missing the move constructor, etc. However, the new diagnostic becomes
(https://godbolt.org/z/9TG3WK75G):
In file included from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/ext/alloc_traits.h:34,
from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:67,
from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/map:60,
from <source>:1:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/alloc_traits.h: In
instantiation of 'static constexpr void
std::allocator_traits<std::allocator<_Up> >::construct(allocator_type&, _Up*,
_Args&& ...) [with _Up = std::pair<const int, C::M>; _Args = {const
std::pair<const int, C::M>&}; _Tp = std::_Rb_tree_node<std::pair<const int,
C::M> >; allocator_type = std::allocator<std::_Rb_tree_node<std::pair<const
int, C::M> > >]':
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:595:32:
required from 'void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_M_construct_node(_Link_type, _Args&& ...) [with _Args = {const
std::pair<const int, C::M>&}; _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
_Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:612:21:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {const
std::pair<const int, C::M>&}; _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
_Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:529:32:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_Alloc_node::operator()(_Arg&&) const [with _Arg = const
std::pair<const int, C::M>&; _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type =
std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:645:18:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_clone_node(_Link_type, _NodeGen&) [with bool _MoveValue =
false; _NodeGen = std::_Rb_tree<int, std::pair<const int, C::M>,
std::_Select1st<std::pair<const int, C::M> >, std::less<int>,
std::allocator<std::pair<const int, C::M> > >::_Alloc_node; _Key = int; _Val =
std::pair<const int, C::M>; _KeyOfValue = std::_Select1st<std::pair<const int,
C::M> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const
int, C::M> >; _Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:1895:47:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_copy(_Link_type, _Base_ptr, _NodeGen&) [with bool
_MoveValues = false; _NodeGen = std::_Rb_tree<int, std::pair<const int, C::M>,
std::_Select1st<std::pair<const int, C::M> >, std::less<int>,
std::allocator<std::pair<const int, C::M> > >::_Alloc_node; _Key = int; _Val =
std::pair<const int, C::M>; _KeyOfValue = std::_Select1st<std::pair<const int,
C::M> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const
int, C::M> >; _Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*;
_Base_ptr = std::_Rb_tree_node_base*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:890:26:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_copy(const std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>&, _NodeGen&) [with bool _MoveValues = false; _NodeGen =
std::_Rb_tree<int, std::pair<const int, C::M>, std::_Select1st<std::pair<const
int, C::M> >, std::less<int>, std::allocator<std::pair<const int, C::M> >
>::_Alloc_node; _Key = int; _Val = std::pair<const int, C::M>; _KeyOfValue =
std::_Select1st<std::pair<const int, C::M> >; _Compare = std::less<int>; _Alloc
= std::allocator<std::pair<const int, C::M> >; _Link_type =
std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:901:29:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_copy(const std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>&) [with _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
_Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:939:23:
required from 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_Rb_tree(const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>&) [with _Key = int; _Val = std::pair<const int, C::M>; _KeyOfValue =
std::_Select1st<std::pair<const int, C::M> >; _Compare = std::less<int>; _Alloc
= std::allocator<std::pair<const int, C::M> >]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_map.h:217:7:
required from here
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/alloc_traits.h:518:28:
error: no matching function for call to 'construct_at(std::pair<const int,
C::M>*&, const std::pair<const int, C::M>&)'
518 | std::construct_at(__p, std::forward<_Args>(__args)...);
| ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_iterator.h:85,
from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_algobase.h:67,
from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:63:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_construct.h:94:5:
note: candidate: 'template<class _Tp, class ... _Args> constexpr decltype
(::new(void*(0)) _Tp) std::construct_at(_Tp*, _Args&& ...)'
94 | construct_at(_Tp* __location, _Args&&... __args)
| ^~~~~~~~~~~~
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_construct.h:94:5:
note: template argument deduction/substitution failed:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_construct.h: In
substitution of 'template<class _Tp, class ... _Args> constexpr decltype
(::new(void*(0)) _Tp) std::construct_at(_Tp*, _Args&& ...) [with _Tp =
std::pair<const int, C::M>; _Args = {const std::pair<const int, C::M>&}]':
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/alloc_traits.h:518:21:
required from 'static constexpr void
std::allocator_traits<std::allocator<_Up> >::construct(allocator_type&, _Up*,
_Args&& ...) [with _Up = std::pair<const int, C::M>; _Args = {const
std::pair<const int, C::M>&}; _Tp = std::_Rb_tree_node<std::pair<const int,
C::M> >; allocator_type = std::allocator<std::_Rb_tree_node<std::pair<const
int, C::M> > >]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:595:32:
required from 'void std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_M_construct_node(_Link_type, _Args&& ...) [with _Args = {const
std::pair<const int, C::M>&}; _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
_Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:612:21:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_create_node(_Args&& ...) [with _Args = {const
std::pair<const int, C::M>&}; _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
_Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:529:32:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_Alloc_node::operator()(_Arg&&) const [with _Arg = const
std::pair<const int, C::M>&; _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type =
std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:645:18:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_clone_node(_Link_type, _NodeGen&) [with bool _MoveValue =
false; _NodeGen = std::_Rb_tree<int, std::pair<const int, C::M>,
std::_Select1st<std::pair<const int, C::M> >, std::less<int>,
std::allocator<std::pair<const int, C::M> > >::_Alloc_node; _Key = int; _Val =
std::pair<const int, C::M>; _KeyOfValue = std::_Select1st<std::pair<const int,
C::M> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const
int, C::M> >; _Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:1895:47:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_copy(_Link_type, _Base_ptr, _NodeGen&) [with bool
_MoveValues = false; _NodeGen = std::_Rb_tree<int, std::pair<const int, C::M>,
std::_Select1st<std::pair<const int, C::M> >, std::less<int>,
std::allocator<std::pair<const int, C::M> > >::_Alloc_node; _Key = int; _Val =
std::pair<const int, C::M>; _KeyOfValue = std::_Select1st<std::pair<const int,
C::M> >; _Compare = std::less<int>; _Alloc = std::allocator<std::pair<const
int, C::M> >; _Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*;
_Base_ptr = std::_Rb_tree_node_base*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:890:26:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_copy(const std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>&, _NodeGen&) [with bool _MoveValues = false; _NodeGen =
std::_Rb_tree<int, std::pair<const int, C::M>, std::_Select1st<std::pair<const
int, C::M> >, std::less<int>, std::allocator<std::pair<const int, C::M> >
>::_Alloc_node; _Key = int; _Val = std::pair<const int, C::M>; _KeyOfValue =
std::_Select1st<std::pair<const int, C::M> >; _Compare = std::less<int>; _Alloc
= std::allocator<std::pair<const int, C::M> >; _Link_type =
std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:901:29:
required from 'std::_Rb_tree_node<_Val>* std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>::_M_copy(const std::_Rb_tree<_Key, _Val, _KeyOfValue,
_Compare, _Alloc>&) [with _Key = int; _Val = std::pair<const int, C::M>;
_KeyOfValue = std::_Select1st<std::pair<const int, C::M> >; _Compare =
std::less<int>; _Alloc = std::allocator<std::pair<const int, C::M> >;
_Link_type = std::_Rb_tree_node<std::pair<const int, C::M> >*]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_tree.h:939:23:
required from 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>::_Rb_tree(const std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare,
_Alloc>&) [with _Key = int; _Val = std::pair<const int, C::M>; _KeyOfValue =
std::_Select1st<std::pair<const int, C::M> >; _Compare = std::less<int>; _Alloc
= std::allocator<std::pair<const int, C::M> >]'
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_map.h:217:7:
required from here
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_construct.h:96:17:
error: use of deleted function 'constexpr std::pair<_T1, _T2>::pair(const
std::pair<_T1, _T2>&) [with _T1 = const int; _T2 = C::M]'
96 | -> decltype(::new((void*)0) _Tp(std::declval<_Args>()...))
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_algobase.h:64:
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_pair.h:195:17:
note: 'constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with
_T1 = const int; _T2 = C::M]' is implicitly deleted because the default
definition would be ill-formed:
195 | constexpr pair(const pair&) = default; ///< Copy constructor
| ^~~~
/opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/stl_pair.h:195:17:
error: use of deleted function 'constexpr C::M::M(const C::M&)'
<source>:13:12: note: 'constexpr C::M::M(const C::M&)' is implicitly declared
as deleted because 'C::M' declares a move constructor or move assignment
operator
13 | struct M
| ^
Compiler returned: 1
This diagnostic is, importantly, missing any reference to _where_ in my code C
is being copied, which is why this took me so long to track down (the create
function sure doesn't look like it's copying!) Instead, it provides me all
these details about map's copy constructor but doesn't actually say that it's
map's copy constructor: I only get a reference to pair's copy constructor
(though at least the call stack has enough info to figure out that it's map's
copy constructor that's the problem).
In short:
1. It would be hugely beneficial if the diagnostic pointing to the originating
line in my code for the problem.
2. It would be nice to have if (a) the diagnostic could explain why I didn't
have a move constructor (though possibly this is over-fitting to this
particular bug) or (b) the diagnostic could not dive into the depths of
libstdc++ and just tell me that std::map<int, M> wasn't copyable because M
wasn't copyable and stop at that point (this one doesn't feel like over-fitting
though).
