__bit_reference: fix -Wdeprecated-copy warnings

[MaskRay]

Summary

Since C++11, [depr.impldec]:

The implicit definition of a copy constructor as defaulted is deprecated
if the class has a user-declared copy assignment operator or a
user-declared destructor.

At clang HEAD, -Wdeprecated-copy (included by -Wextra) will warn on such instances.

[MaskRay]

Ping :)

[mclow.lists]

Is this related to https://bugs.llvm.org/show_bug.cgi?id=44145 ?

[MaskRay]

In D71096#1781892, @mclow.lists wrote:

Is this related to https://bugs.llvm.org/show_bug.cgi?id=44145 ?

Yes. It fixes the warnings, when I run ninja all with a latest clang. Thankfully there are not too many #include <bitset> in the llvm code base, so the warnings are not overwhelming.

/b/sanitizer-x86_64-linux/build/llvm-project/libcxx/include/bitset:628:17: note: in implicit copy constructor for 'std::__1::__bit_reference<std::__1::__bitset<0, 0>, true>' first required here
        {return reference(0, 1);}

[MaskRay]

libcxx/include/__bit_reference

50

I guess this file needs to be C++03 compatible, so I don't use = default.

150

It seems this isn't defined, so I just delete it.

[dblaikie]

libcxx/include/__bit_reference

150

Perhaps it was intended to be declared-but-not-defined to disable the use of copy assignment for this type? (in the days before "= delete" this is how things were done - declare the function privately without a definition)

[EricWF]

LGTM given the use of = default and = delete.

libcxx/include/__bit_reference

51

= default is C++03 compatible because we allow extensions.

Please go back to using = default.

150

Yes, please = delete this.

(As mentioned above = delete can be used in C++03 as well).

[EricWF]

LGTM once the changes land. Let's shut this warning up.

[EricWF]

libcxx/include/__bit_reference

150

Oh, you'll have to = default the copy constructor for __bit_const_reference after doing this.

[MaskRay]

Use = default and = delete;

libcxx/include/__bit_reference

class __bit_reference
     __storage_pointer __seg_;
     __storage_type    __mask_;
 
     friend typename _Cp::__self;
 
     friend class __bit_const_reference<_Cp>;
     friend class __bit_iterator<_Cp, false>;
 public:
+    _LIBCPP_INLINE_VISIBILITY
+    __bit_reference(const __bit_reference&) = default;
+
     _LIBCPP_INLINE_VISIBILITY operator bool() const _NOEXCEPT
         {return static_cast<bool>(*__seg_ & __mask_);}
     _LIBCPP_INLINE_VISIBILITY bool operator ~() const _NOEXCEPT
         {return !static_cast<bool>(*this);}
 
     _LIBCPP_INLINE_VISIBILITY
     __bit_reference& operator=(bool __x) _NOEXCEPT
     {
class __bit_const_reference
 
     __storage_pointer        __seg_;
     __storage_type __mask_;
 
     friend typename _Cp::__self;
     friend class __bit_iterator<_Cp, true>;
 public:
     _LIBCPP_INLINE_VISIBILITY
+    __bit_const_reference(const __bit_const_reference&) = default;
+
+    _LIBCPP_INLINE_VISIBILITY
     __bit_const_reference(const __bit_reference<_Cp>& __x) _NOEXCEPT
         : __seg_(__x.__seg_), __mask_(__x.__mask_) {}
 
     _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR operator bool() const _NOEXCEPT
         {return static_cast<bool>(*__seg_ & __mask_);}
 
     _LIBCPP_INLINE_VISIBILITY __bit_iterator<_Cp, true> operator&() const _NOEXCEPT
         {return __bit_iterator<_Cp, true>(__seg_, static_cast<unsigned>(__libcpp_ctz(__mask_)));}
 private:
     _LIBCPP_INLINE_VISIBILITY
     _LIBCPP_CONSTEXPR
     __bit_const_reference(__storage_pointer __s, __storage_type __m) _NOEXCEPT
         : __seg_(__s), __mask_(__m) {}
 
-    __bit_const_reference& operator=(const __bit_const_reference& __x);
+    __bit_const_reference& operator=(const __bit_const_reference&) = delete;
 };
 
 // find
 
 template <class _Cp, bool _IsConst>
 __bit_iterator<_Cp, _IsConst>
 __find_bool_true(__bit_iterator<_Cp, _IsConst> __first, typename _Cp::size_type __n)
 {