Fix handling of output iterators in format_to_n (#1506)

include/fmt/format.h

@@ -395,6 +395,7 @@ template <typename OutputIt> class truncating_iterator_base {
 
  public:
   using iterator_category = std::output_iterator_tag;
+  using value_type = typename std::iterator_traits<OutputIt>::value_type;
   using difference_type = void;
   using pointer = void;
   using reference = void;
@@ -415,12 +416,10 @@ class truncating_iterator;
 template <typename OutputIt>
 class truncating_iterator<OutputIt, std::false_type>
     : public truncating_iterator_base<OutputIt> {
-  using traits = std::iterator_traits<OutputIt>;
-
-  mutable typename traits::value_type blackhole_;
+  mutable typename truncating_iterator_base<OutputIt>::value_type blackhole_;
 
  public:
-  using value_type = typename traits::value_type;
+  using value_type = typename truncating_iterator_base<OutputIt>::value_type;
 
   truncating_iterator(OutputIt out, std::size_t limit)
       : truncating_iterator_base<OutputIt>(out, limit) {}
@@ -445,13 +444,11 @@ template <typename OutputIt>
 class truncating_iterator<OutputIt, std::true_type>
     : public truncating_iterator_base<OutputIt> {
  public:
-  using value_type = typename OutputIt::container_type::value_type;
-
   truncating_iterator(OutputIt out, std::size_t limit)
       : truncating_iterator_base<OutputIt>(out, limit) {}
 
-  truncating_iterator& operator=(value_type val) {
-    if (this->count_++ < this->limit_) this->out_ = val;
+  template <typename T> truncating_iterator& operator=(T val) {
+    if (this->count_++ < this->limit_) *this->out_++ = val;
     return *this;
   }
 

test/format-test.cc

@@ -2080,6 +2080,33 @@ TEST(FormatTest, WideFormatToN) {
   EXPECT_EQ(L"BC x", fmt::wstring_view(buffer, 4));
 }
 
+struct test_output_iterator {
+  char* data;
+
+  using iterator_category = std::output_iterator_tag;
+  using value_type = void;
+  using difference_type = void;
+  using pointer = void;
+  using reference = void;
+
+  test_output_iterator& operator++() {
+    ++data;
+    return *this;
+  }
+  test_output_iterator operator++(int) {
+    auto tmp = *this;
+    ++data;
+    return tmp;
+  }
+  char& operator*() { return *data; }
+};
+
+TEST(FormatTest, FormatToNOutputIterator) {
+  char buf[10] = {};
+  fmt::format_to_n(test_output_iterator{buf}, 10, "{}", 42);
+  EXPECT_STREQ(buf, "42");
+}
+
 #if FMT_USE_CONSTEXPR
 struct test_arg_id_handler {
   enum result { NONE, EMPTY, INDEX, NAME, ERROR };