[libc++] Make common_iterator's data member private

[ldionne]

Fixes #71498

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Louis Dionne (ldionne)

Changes

Fixes #71498

---

Full diff: https://github.com/llvm/llvm-project/pull/72564.diff

1 Files Affected:

• (modified) libcxx/include/__iterator/common_iterator.h (+4-1)

diff --git a/libcxx/include/__iterator/common_iterator.h b/libcxx/include/__iterator/common_iterator.h
index 95e248d83f4b460..7750cf0f7638d6b 100644
--- a/libcxx/include/__iterator/common_iterator.h
+++ b/libcxx/include/__iterator/common_iterator.h
@@ -63,9 +63,12 @@ class common_iterator {
     iter_value_t<_Iter> __value_;
   };
 
-public:
   variant<_Iter, _Sent> __hold_;
+  template<input_or_output_iterator _OtherIter, sentinel_for<_Iter> _OtherSent>
+    requires (!same_as<_OtherIter, _OtherSent> && copyable<_OtherIter>)
+  friend class common_iterator;
 
+public:
   _LIBCPP_HIDE_FROM_ABI common_iterator() requires default_initializable<_Iter> = default;
 
   _LIBCPP_HIDE_FROM_ABI constexpr common_iterator(_Iter __i) : __hold_(in_place_type<_Iter>, _VSTD::move(__i)) {}

[hawkinsw]

Not that my opinion matters for much, but I see nothing wrong with this approach.