Make truncating_iterator an output_iterator (#2158)

test/format-test.cc

@@ -12,9 +12,11 @@
 #include <climits>
 #include <cmath>
 #include <cstring>
+#include <iterator>
 #include <list>
 #include <memory>
 #include <string>
+#include <type_traits>
 
 // Check if fmt/format.h compiles with windows.h included before it.
 #ifdef _WIN32
@@ -157,6 +159,24 @@ TEST(IteratorTest, TruncatingIterator) {
   EXPECT_EQ(it.base(), p + 1);
 }
 
+
+TEST(IteratorTest, TruncatingIteratorDefaultConstruct) {
+  static_assert(
+      std::is_default_constructible<fmt::detail::truncating_iterator<char*>>::value,
+      "");
+  
+  fmt::detail::truncating_iterator<char*> it;
+  EXPECT_EQ(nullptr, it.base());
+  EXPECT_EQ(std::size_t{0}, it.count());
+}
+
+#ifdef __cpp_lib_ranges
+TEST(IteratorTest, TruncatingIteratorOutputIterator) {
+  static_assert(std::output_iterator<fmt::detail::truncating_iterator<char*>,
+      char>);
+}
+#endif
+
 TEST(IteratorTest, TruncatingBackInserter) {
   std::string buffer;
   auto bi = std::back_inserter(buffer);