[llvm-bugs] [Bug 27442] New: std::vector emplace_back instantiates implicitly-deleted copy constructor

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27442

            Bug ID: 27442
           Summary: std::vector emplace_back instantiates
                    implicitly-deleted copy constructor
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: dexonsmith at apple.com
                CC: dblaikie at gmail.com, george.burgess.iv at gmail.com,
                    llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

Created attachment 16242
  --> https://llvm.org/bugs/attachment.cgi?id=16242&action=edit
t.cpp

Using r266855, given:
--
#include <vector>

struct NonCopyable {
  NonCopyable(int);
  NonCopyable(NonCopyable&&);
};

template <class T> struct Wrapper {
  T value;
  Wrapper(int value) : value(value) {}
  Wrapper(Wrapper &&X) : value(std::move(X.value)) {}
  Wrapper(const Wrapper &X) : value(X.value) {}
};

void foo() {
  std::vector<Wrapper<NonCopyable>> vector;
  vector.emplace_back(5);
}
--

I get the following:
--
$ path/to/clang -nostdinc++ -isystem path/to/libcxx -std=c++11 t.cpp
-fsyntax-only
t.cpp:12:31: error: call to implicitly-deleted copy constructor of
'NonCopyable'
  Wrapper(const Wrapper &X) : value(X.value) {}
                              ^     ~~~~~~~
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/memory:1748:31:
note: in instantiation of member function 'Wrapper<NonCopyable>::Wrapper'
requested here
            ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                              ^
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/memory:1656:18:
note: in instantiation of function template specialization
'std::__1::allocator<Wrapper<NonCopyable>
      >::construct<Wrapper<NonCopyable>, const Wrapper<NonCopyable> &>'
requested here
            {__a.construct(__p, _VSTD::forward<_Args>(__args)...);}
                 ^
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/memory:1502:14:
note: in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<Wrapper<NonCopyable> >
>::__construct<Wrapper<NonCopyable>, const Wrapper<NonCopyable> &>' requested
here
            {__construct(__has_construct<allocator_type, _Tp*, _Args...>(),
             ^
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/memory:1618:17:
note: in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<Wrapper<NonCopyable> >
>::construct<Wrapper<NonCopyable>, const Wrapper<NonCopyable> &>' requested
here
                construct(__a, _VSTD::__to_raw_pointer(__end2-1),
_VSTD::move_if_noexcept(*--__end1));
                ^
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/vector:892:21:
note: in instantiation of function template specialization
      'std::__1::allocator_traits<std::__1::allocator<Wrapper<NonCopyable> >
>::__construct_backward<Wrapper<NonCopyable> *>' requested here
    __alloc_traits::__construct_backward(this->__alloc(), this->__begin_,
this->__end_, __v.__begin_);
                    ^
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/vector:1629:5:
note: in instantiation of member function
'std::__1::vector<Wrapper<NonCopyable>,
      std::__1::allocator<Wrapper<NonCopyable> > >::__swap_out_circular_buffer'
requested here
    __swap_out_circular_buffer(__v);
    ^
/Users/dexonsmith/data/llvm/llvm.org/projects/libcxx/include/vector:1648:9:
note: in instantiation of function template specialization
'std::__1::vector<Wrapper<NonCopyable>,
      std::__1::allocator<Wrapper<NonCopyable> >
>::__emplace_back_slow_path<int>' requested here
        __emplace_back_slow_path(_VSTD::forward<_Args>(__args)...);
        ^
t.cpp:17:10: note: in instantiation of function template specialization
'std::__1::vector<Wrapper<NonCopyable>,
std::__1::allocator<Wrapper<NonCopyable> > >::emplace_back<int>' requested here
  vector.emplace_back(5);
         ^
t.cpp:5:3: note: copy constructor is implicitly deleted because 'NonCopyable'
has a user-declared move constructor
  NonCopyable(NonCopyable&&);
  ^
1 error generated.
--

Commenting out `Wrapper(const Wrapper &X)` will let it compile.  It's not clear
why having the extra (implicitly deleted) constructor around causes
emplace_back to fail to compile.

This compile error scared me enough that I committed r266909 to stop using
std::vector on types that are expensive to copy.

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