Add a test to cover #59 so that it does not pop up again.

Fixes #60.

test/CMakeLists.txt

@@ -49,6 +49,7 @@ add_executable(
     compile_tests
     compile_tests_main.cpp
     compile_seq_cont_rvalue_constrained_pop_back.cpp
+    compile_sfinae_path_mutable_iterator.cpp
 )
 target_link_libraries(compile_tests stl_interfaces)
 if (clang_on_linux)

test/Jamfile.v2

@@ -26,3 +26,4 @@ run random_access.cpp ;
 run static_vec.cpp ;
 
 compile compile_seq_cont_rvalue_constrained_pop_back.cpp ;
+compile compile_sfinae_path_mutable_iterator.cpp ;

test/compile_sfinae_path_mutable_iterator.cpp

@@ -0,0 +1,77 @@
+// Copyright (C) 2022 T. Zachary Laine
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+#include <boost/stl_interfaces/iterator_interface.hpp>
+#include <type_traits>
+#include <utility>
+
+struct node
+{
+    using value_type = std::pair<int, int>;
+
+    value_type kv_;
+    node * next_;
+};
+
+template<typename Node>
+struct iterator : boost::stl_interfaces::iterator_interface<
+                      iterator<Node>,
+                      std::forward_iterator_tag,
+                      Node>
+{
+    using value_type = typename Node::value_type;
+
+    constexpr iterator() noexcept = default;
+    constexpr explicit iterator(Node * it) : it_{it} {}
+
+    template<
+        typename Node2,
+        typename Enable = std::enable_if_t<
+            std::is_convertible_v<Node2 *, Node *> &&
+            !std::is_const_v<Node2> && std::is_const_v<Node>>>
+    constexpr iterator(iterator<Node2> other) noexcept : it_{other.it_}
+    {}
+
+    constexpr value_type const & operator*() const noexcept { return it_->kv_; }
+
+    template<
+        typename T = Node,
+        typename std::enable_if_t<!std::is_const_v<T>, bool> = true>
+    constexpr value_type & operator*() noexcept
+    {
+        return it_->kv_;
+    }
+
+    constexpr iterator & operator++() noexcept
+    {
+        it_ = it_->next_;
+        return *this;
+    }
+
+    using base_type = boost::stl_interfaces::iterator_interface<
+        iterator<Node>,
+        std::forward_iterator_tag,
+        Node>;
+    using base_type::operator++;
+
+    friend constexpr bool operator==(iterator lhs, iterator rhs) noexcept
+    {
+        return lhs.it_ == rhs.it_;
+    }
+
+private:
+    Node * it_;
+
+    template<typename Node2>
+    friend struct iterator;
+};
+
+void compile_sfinae_path_mutable_iterator()
+{
+    auto it = iterator<node>{};
+    if (it->first == 42) {
+        it->second = 13;
+    }
+}

test/compile_tests_main.cpp

@@ -4,8 +4,10 @@
 // accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 void compile_seq_cont_constrained_pop_back();
+void compile_sfinae_path_mutable_iterator();
 
 int main()
 {
     compile_seq_cont_constrained_pop_back();
+    compile_sfinae_path_mutable_iterator();
 }