I was improving Chrome code by removing our "linked_ptr" custom type, and hit a situation with bad diagnostics from Clang (see https://chromium-review.googlesource.com/c/chromium/src/+/1368667). I have a reduced case.
Suppose we have code that tries to iterate over a map:
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#include <map>
#include <memory>
int main() {
using intmap = std::map<std::string, std::unique_ptr<int>>;
intmap m;
for (const std::pair<std::string,
std::unique_ptr<int>>& map_pair : m) {
// do something with map_pair
}
return 0;
}
==================================================
Note that it does it *incorrectly*. intmap::value_type is std::pair<**const** std::string, ...>. Clang's diagnostics are helpful in this case:
==================================================
<source>:10:49: error: no viable conversion from 'pair<const basic_string<...>, [...]>' to 'const pair<basic_string<...>, [...]>'
std::unique_ptr<int>>& map_pair : m) {
^ ~
1 error generated.
==================================================
However, suppose that the type used is a map of a map of a move-only type:
==================================================
#include <map>
#include <memory>
int main() {
using intmap = std::map<std::string,
std::unique_ptr<int>>;
std::map<std::string, intmap> m2;
for (const std::pair<std::string, intmap>& map_pair : m2) {
// do something with map_pair
}
return 0;
}
==================================================
In this case, Clang's diagnostics are completely useless, blaming only the stdlib and not indicating the error in the user code:
==================================================
In file included from <source>:1:
In file included from /opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/map:60:
In file included from /opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:64:
In file included from /opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/allocator.h:46:
In file included from /opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/x86_64-linux-gnu/bits/c++allocator.h:33:
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/ext/new_allocator.h:136:23: error: call to implicitly-deleted copy constructor of 'std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >'
{ ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/alloc_traits.h:475:8: note: in instantiation of function template specialization '__gnu_cxx::new_allocator<std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::construct<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, const std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > &>' requested here
{ __a.construct(__p, std::forward<_Args>(__args)...); }
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:626:23: note: in instantiation of function template specialization 'std::allocator_traits<std::allocator<std::_Rb_tree_node<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > > >::construct<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, const std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > &>' requested here
_Alloc_traits::construct(_M_get_Node_allocator(),
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:643:4: note: in instantiation of function template specialization 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_M_construct_node<const std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > &>' requested here
_M_construct_node(__tmp, std::forward<_Args>(__args)...);
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:556:18: note: in instantiation of function template specialization 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_M_create_node<const std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > &>' requested here
{ return _M_t._M_create_node(_GLIBCXX_FORWARD(_Arg, __arg)); }
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:666:23: note: in instantiation of function template specialization 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_Alloc_node::operator()<const std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > &>' requested here
_Link_type __tmp = __node_gen(*__x->_M_valptr());
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:1818:21: note: in instantiation of function template specialization 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_M_clone_node<std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_Alloc_node>' requested here
_Link_type __top = _M_clone_node(__x, __node_gen);
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:875:24: note: in instantiation of function template specialization 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_M_copy<std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_Alloc_node>' requested here
_Link_type __root = _M_copy(__x._M_begin(), _M_end(), __gen);
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:886:9: note: in instantiation of function template specialization 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_M_copy<std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_Alloc_node>' requested here
return _M_copy(__x, __an);
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_tree.h:924:16: note: in instantiation of member function 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_M_copy' requested here
_M_root() = _M_copy(__x);
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_map.h:199:7: note: in instantiation of member function 'std::_Rb_tree<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >, std::_Select1st<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > >, std::less<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > > > >::_Rb_tree' requested here
map(const map&) = default;
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_pair.h:292:17: note: explicitly defaulted function was implicitly deleted here
constexpr pair(const pair&) = default;
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/stl_pair.h:204:11: note: copy constructor of 'pair<const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::unique_ptr<int, std::default_delete<int> > >' is implicitly deleted because field 'second' has a deleted copy constructor
_T2 second; /// @c second is a copy of the second object
^
/opt/compiler-explorer/gcc-7.3.0/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/bits/unique_ptr.h:388:7: note: 'unique_ptr' has been explicitly marked deleted here
unique_ptr(const unique_ptr&) = delete;
^
1 error generated.
==================================================
FYI, GCC trunk and MSVC 19.16 fail in similar ways. icc does slightly better but not by much.
Clang does really well for the singly-nested case; it should give similar diagnostics in this second case.
