Fix wxString iterator comparison in C++20

In C++20 the reverse comparison operators are also considered when
searching for the operator to use and a wrong operator was selected for
comparisons between iterator and const_iterator, that would result in an
infinite recursion at run-time.

Fix this, thanks to the nice gcc 10 warning about it, by explicitly
defining the operators for this overload set too instead of relying on
implicit conversions.

Although not all these overloads are necessary, and they are only
necessary in C++20, it seems better to define all of them and always
just to be perfectly explicit and clear, as this code is not exactly
simple to follow.

include/wx/string.h

@@ -884,10 +884,18 @@ public:
       const_iterator operator-(ptrdiff_t n) const
         { return const_iterator(str(), wxStringOperations::AddToIter(m_cur, -n)); }
 
-      // Notice that comparison operators taking non-const iterator are not
+      // Until C++20 we could avoid defining these comparison operators because
-      // needed here because of the implicit conversion from non-const iterator
+      // the implicit conversion from iterator to const_iterator was used to
-      // to const ones ensure that the versions for const_iterator declared
+      // reuse the operators defined inside WX_STR_ITERATOR_IMPL. However in
-      // inside WX_STR_ITERATOR_IMPL can be used.
+      // C++20 the operator overloads with reversed arguments would be used
+      // instead, resulting in infinite recursion, so we do need them and, just
+      // for consistency, define them in all cases.
+      bool operator==(const iterator& i) const;
+      bool operator!=(const iterator& i) const;
+      bool operator<(const iterator& i) const;
+      bool operator>(const iterator& i) const;
+      bool operator<=(const iterator& i) const;
+      bool operator>=(const iterator& i) const;
 
   private:
       // for internal wxString use only:
@@ -954,10 +962,13 @@ public:
       const_iterator operator-(ptrdiff_t n) const
         { return const_iterator(wxStringOperations::AddToIter(m_cur, -n)); }
 
-      // As in UTF-8 case above, we don't need comparison operators taking
+      // See comment for comparison operators in the UTF-8 case above.
-      // iterator because we have an implicit conversion from iterator to
+      bool operator==(const iterator& i) const;
-      // const_iterator so the operators declared by WX_STR_ITERATOR_IMPL will
+      bool operator!=(const iterator& i) const;
-      // be used.
+      bool operator<(const iterator& i) const;
+      bool operator>(const iterator& i) const;
+      bool operator<=(const iterator& i) const;
+      bool operator>=(const iterator& i) const;
 
   private:
       // for internal wxString use only:
@@ -3934,6 +3945,19 @@ inline bool operator!=(const wxString& s, wchar_t c) { return !s.IsSameAs(c); }
 
 
 // wxString iterators comparisons
+inline bool wxString::const_iterator::operator==(const iterator& i) const
+    { return *this == const_iterator(i); }
+inline bool wxString::const_iterator::operator!=(const iterator& i) const
+    { return *this != const_iterator(i); }
+inline bool wxString::const_iterator::operator<(const iterator& i) const
+    { return *this < const_iterator(i); }
+inline bool wxString::const_iterator::operator>(const iterator& i) const
+    { return *this > const_iterator(i); }
+inline bool wxString::const_iterator::operator<=(const iterator& i) const
+    { return *this <= const_iterator(i); }
+inline bool wxString::const_iterator::operator>=(const iterator& i) const
+    { return *this >= const_iterator(i); }
+
 inline bool wxString::iterator::operator==(const const_iterator& i) const
     { return i == *this; }
 inline bool wxString::iterator::operator!=(const const_iterator& i) const