Fix zip/zip_with distance_to (#1776)

include/range/v3/view/zip_with.hpp

@@ -282,7 +282,11 @@ namespace ranges
             {
                 // Return the smallest distance (in magnitude) of any of the iterator
                 // pairs. This is to accommodate zippers of sequences of different length.
-                if(0 < std::get<0>(that.its_) - std::get<0>(its_))
+                auto first_size = std::get<0>(that.its_) - std::get<0>(its_);
+                if(first_size == 0)
+                    return static_cast<difference_type>(0);
+
+                if(0 < first_size)
                     return tuple_foldl(
                         tuple_transform(its_, that.its_, detail::distance_to),
                         (std::numeric_limits<difference_type>::max)(),

test/view/zip.cpp

@@ -302,6 +302,29 @@ int main()
 
         has_cardinality<cardinality::unknown>(rng);
     }
+
+    {
+        std::vector<int> v0{1, 2, 3};
+        std::vector<int> v1{};
+
+        auto rng0 = views::zip(v0, v1);
+        auto rng1 = views::zip(v1, v0);
+
+        CHECK(ranges::distance(ranges::begin(rng0), ranges::end(rng0)) == 0);
+        CHECK(ranges::distance(ranges::begin(rng1), ranges::end(rng1)) == 0);
+        CHECK(ranges::distance(ranges::end(rng0), ranges::begin(rng0)) == 0);
+        CHECK(ranges::distance(ranges::end(rng1), ranges::begin(rng1)) == 0);
+    }
+
+    {
+        std::vector<int> v0{1, 2, 3};
+        std::vector<int> v1{1, 2, 3, 4, 5};
+
+        auto rng = views::zip(v0, v1);
+
+        CHECK(ranges::distance(ranges::begin(rng), ranges::end(rng)) == 3);
+        CHECK(ranges::distance(ranges::end(rng), ranges::begin(rng)) == -3);
+    }
 
     return test_result();
 }