libstdc++: Fix regression in std::stable_sort

The recent change to split out the cold path of std::stable_sort caused
a regression for some Qt code. The problem is that the library now adds
a value of type ptrdiff_t to the iterator, which is ambiguous with
-pedantic. The addition could either convert the iterator to a built-in
pointer and add the ptrdiff_t to that, or it could convert the ptrdiff_t
to the iterator's difference_type and use the iterator's own operator+.

The fix is to cast the ptrdiff_t value to the difference type first.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (__stable_sort): Cast size to
	iterator's difference type.
	* testsuite/25_algorithms/stable_sort/4.cc: New test.

2 files changed, 54 insertions, 2 deletions

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index c607805..57fa1c1 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -5026,8 +5026,9 @@ _GLIBCXX_BEGIN_NAMESPACE_ALGO
       _TmpBuf __buf(__first, (__last - __first + 1) / 2);
 
       if (__builtin_expect(__buf.requested_size() == __buf.size(), true))
-	std::__stable_sort_adaptive(__first, __first + __buf.size(), __last,
-				    __buf.begin(), __comp);
+	std::__stable_sort_adaptive(__first,
+				    __first + _DistanceType(__buf.size()),
+				    __last, __buf.begin(), __comp);
       else if (__builtin_expect(__buf.begin() == 0, false))
 	std::__inplace_stable_sort(__first, __last, __comp);
       else
diff --git a/libstdc++-v3/testsuite/25_algorithms/stable_sort/4.cc b/libstdc++-v3/testsuite/25_algorithms/stable_sort/4.cc
new file mode 100644
index 0000000..b7bda4e
--- /dev/null
+++ b/libstdc++-v3/testsuite/25_algorithms/stable_sort/4.cc
@@ -0,0 +1,51 @@
+// { dg-options "-pedantic" }
+// { dg-do compile }
+
+#include <algorithm>
+
+/* This type is reduced from QTypedArrayData::iterator which has an implicit
+ * conversion to its pointer type and a difference type of int.
+ * The expression Iter() + ptrdiff_t(0) is ambiguous with -pedantic because it
+ * could either convert the RHS to int and use Iter::operator+(int)
+ * or it could convert the LHS to pointer and use built-in pointer arithmetic.
+ */
+struct Iter
+{
+  struct value_type { bool operator<(value_type) const; };
+  typedef value_type* pointer;
+  typedef value_type& reference;
+  typedef std::random_access_iterator_tag iterator_category;
+  typedef int difference_type;
+
+  reference operator*() const;
+  pointer operator->() const;
+
+  reference operator[](difference_type) const;
+
+  Iter& operator++();
+  Iter& operator--();
+  Iter operator++(int);
+  Iter operator--(int);
+
+  Iter& operator+=(difference_type);
+  Iter& operator-=(difference_type);
+
+  Iter operator+(difference_type) const;
+  Iter operator-(difference_type) const;
+
+  difference_type operator-(Iter) const;
+
+  operator pointer() const; // XXX this causes the ambiguity
+
+  bool operator==(Iter) const;
+  bool operator!=(Iter) const;
+
+  bool operator<(Iter) const;
+};
+
+Iter operator+(Iter::difference_type, Iter);
+
+int main()
+{
+  std::stable_sort(Iter(), Iter());
+}