[glibmm] Revert "ustring_Iterator: Declare the copy constructor =default"

[Kjell Ahlstedt <kjellahl src gnome org>]

commit cad27382259efc0f1c6e0c9d97ae8f426200fa1a
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed May 4 16:45:40 2022 +0200

    Revert "ustring_Iterator: Declare the copy constructor =default"
    
    This reverts commit 7b811a0be824675f31a422d40a75bbb5d10e32ad.
    It broke ABI. See #98

 glib/glibmm/ustring.h | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)
---
diff --git a/glib/glibmm/ustring.h b/glib/glibmm/ustring.h
index 594a2f29..41165f58 100644
--- a/glib/glibmm/ustring.h
+++ b/glib/glibmm/ustring.h
@@ -198,14 +198,7 @@ public:
   using pointer = void;
 
   inline ustring_Iterator();
-  // A std::string::iterator can be copied to a std::string::const_iterator.
-  template <typename T2, typename = typename std::enable_if<
-    std::is_same<std::string::const_iterator, T>::value &&
-    std::is_same<std::string::iterator, T2>::value, T2>::type>
-  inline ustring_Iterator(const ustring_Iterator<T2>& other)
-  : pos_(other.base())
-  { }
-  ustring_Iterator(const ustring_Iterator& other) = default;
+  inline ustring_Iterator(const ustring_Iterator<std::string::iterator>& other);
   ustring_Iterator& operator=(const ustring_Iterator& other) = default;
 
   inline value_type operator*() const;
@@ -1073,6 +1066,12 @@ inline ustring_Iterator<T>::ustring_Iterator() : pos_()
 {
 }
 
+template <class T>
+inline ustring_Iterator<T>::ustring_Iterator(const ustring_Iterator<std::string::iterator>& other)
+: pos_(other.base())
+{
+}
+
 template <class T>
 inline typename ustring_Iterator<T>::value_type ustring_Iterator<T>::operator*() const
 {