From cc063d5395a25db9a675f6012e4c204057485a31 Mon Sep 17 00:00:00 2001
From: Casey Bodley <cbodley@redhat.com>
Date: Fri, 11 Feb 2022 18:26:22 -0500
Subject: [PATCH] buffer: constrain conversions of buffers_t iterators

prevent conversions from const_iterator -> iterator to resolve a c++20
compilation error with clang 13:

ceph/src/include/buffer.h:537:18: error: use of overloaded operator '==' is ambiguous (with operand types 'ceph::buffer::list::buffers_t::const_iterator' (aka 'buffers_iterator<const ceph::buffer::ptr_node>') and 'ceph::buffer::list::buffers_t::iterator' (aka 'buffers_iterator<ceph::buffer::ptr_node>'
))
        _root.next = it == end() ? &item : _root.next;
                     ~~ ^  ~~~~~
ceph/src/include/buffer.h:471:7: note: candidate function
        bool operator==(const buffers_iterator& rhs) const {
             ^
ceph/src/include/buffer.h:471:7: note: candidate function (with reversed parameter order)

Signed-off-by: Casey Bodley <cbodley@redhat.com>
---
 src/include/buffer.h | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/src/include/buffer.h b/src/include/buffer.h
index 8a28d846d9d..5e3fdf4b40b 100644
--- a/src/include/buffer.h
+++ b/src/include/buffer.h
@@ -440,7 +440,13 @@ struct error_code;
 	buffers_iterator(U* const p)
 	  : cur(p) {
 	}
-	template <class U>
+	// copy constructor
+	buffers_iterator(const buffers_iterator<T>& other)
+	  : cur(other.cur) {
+	}
+	// converting constructor, from iterator -> const_iterator only
+	template <class U, typename std::enable_if<
+	    std::is_const<T>::value && !std::is_const<U>::value, int>::type = 0>
 	buffers_iterator(const buffers_iterator<U>& other)
 	  : cur(other.cur) {
 	}
@@ -475,11 +481,6 @@ struct error_code;
 	bool operator!=(const buffers_iterator& rhs) const {
 	  return !(*this==rhs);
 	}
-
-	using citer_t = buffers_iterator<typename std::add_const<T>::type>;
-	operator citer_t() const {
-	  return citer_t(cur);
-	}
       };
 
       typedef buffers_iterator<const ptr_node> const_iterator;
-- 
2.39.5