fix: 'format_to_n' compiles 'std::back_inserter' arguments

std::back_insert_iterators model the OutputIterator concept but differ considerably in their traits and behavior. In particular the former made compilation to fail when format_to_n is given a back_inserter as first argument. The emulation of an OutputIterator is not perfect due to the behavioural differences of back_insert_iterators (e.g. assignment always implies increment) but good enough to be used within fmt's machinery.

include/fmt/format.h

@@ -740,8 +740,12 @@ class counting_iterator {
 
 // An output iterator that truncates the output and counts the number of objects
 // written to it.
+template <typename OutputIt, typename Enable = typename std::is_void<
+    typename std::iterator_traits<OutputIt>::value_type>::type>
+class truncating_iterator;
+
 template <typename OutputIt>
-class truncating_iterator {
+class truncating_iterator<OutputIt, std::false_type> {
  private:
   typedef std::iterator_traits<OutputIt> traits;
 
@@ -779,6 +783,40 @@ class truncating_iterator {
   reference operator*() const { return count_ < limit_ ? *out_ : blackhole_; }
 };
 
+template <typename OutputIt>
+class truncating_iterator<OutputIt, std::true_type> {
+ private:
+  OutputIt out_;
+  std::size_t limit_;
+  std::size_t count_;
+
+ public:
+  typedef std::output_iterator_tag iterator_category;
+  typedef typename OutputIt::container_type::value_type value_type;
+  typedef void difference_type;
+  typedef void pointer;
+  typedef void reference;
+  typedef truncating_iterator _Unchecked_type;  // Mark iterator as checked.
+
+  truncating_iterator(OutputIt out, std::size_t limit)
+    : out_(out), limit_(limit), count_(0) {}
+
+  OutputIt base() const { return out_; }
+  std::size_t count() const { return count_; }
+
+  truncating_iterator& operator=(value_type val) {
+    if (count_++ < limit_)
+      out_ = val;
+    return *this;
+  }
+
+  truncating_iterator& operator++() { return *this; }
+
+  truncating_iterator& operator++(int) { return *this; }
+
+  truncating_iterator& operator*() { return *this; }
+};
+
 // Returns true if value is negative, false otherwise.
 // Same as (value < 0) but doesn't produce warnings if T is an unsigned type.
 template <typename T>

test/format-test.cc

@@ -12,6 +12,7 @@
 #include <cstring>
 #include <list>
 #include <memory>
+#include <string>
 #include <stdint.h>
 
 // Check if fmt/format.h compiles with windows.h included before it.
@@ -185,6 +186,17 @@ TEST(IteratorTest, TruncatingIterator) {
   EXPECT_EQ(it.base(), p + 1);
 }
 
+TEST(IteratorTest, TruncatingBackInserter) {
+  std::string buffer;
+  auto bi = std::back_inserter(buffer);
+  fmt::internal::truncating_iterator<decltype(bi)> it(bi, 2);
+  *it++ = '4';
+  *it++ = '2';
+  *it++ = '1';
+  EXPECT_EQ(buffer.size(), 2);
+  EXPECT_EQ(buffer, "42");
+}
+
 TEST(MemoryBufferTest, Ctor) {
   basic_memory_buffer<char, 123> buffer;
   EXPECT_EQ(static_cast<size_t>(0), buffer.size());