Optimize copy_str for counting_iterator

include/fmt/format.h

@@ -443,13 +443,17 @@ class counting_iterator {
     ++count_;
     return *this;
   }
-
   counting_iterator operator++(int) {
     auto it = *this;
     ++*this;
     return it;
   }
 
+  friend counting_iterator operator+(counting_iterator it, difference_type n) {
+    it.count_ += static_cast<size_t>(n);
+    return it;
+  }
+
   value_type operator*() const { return {}; }
 };
 
@@ -583,6 +587,12 @@ OutputIt copy_str(InputIt begin, InputIt end, OutputIt it) {
                         [](char c) { return static_cast<char8_type>(c); });
 }
 
+template <typename Char, typename InputIt>
+inline counting_iterator copy_str(InputIt begin, InputIt end,
+                                  counting_iterator it) {
+  return it + (end - begin);
+}
+
 #ifndef FMT_USE_GRISU
 #  define FMT_USE_GRISU 1
 #endif

test/format-test.cc

@@ -147,6 +147,7 @@ TEST(IteratorTest, CountingIterator) {
   auto prev = it++;
   EXPECT_EQ(prev.count(), 0);
   EXPECT_EQ(it.count(), 1);
+  EXPECT_EQ((it + 41).count(), 42);
 }
 
 TEST(IteratorTest, TruncatingIterator) {