Disallow writing a wide string to a char stream and vice versa, because there is no good way to implement these function without knowing the encodings and what std::ostream doesn't make any sense.

format.cc

@@ -73,15 +73,12 @@ inline int IsInf(double x) { return !_finite(x); }
 #define FMT_SNPRINTF sprintf_s
 
 #endif  // _MSC_VER
+}
 
-template <typename Char>
-struct CharTraits;
-
-template <>
-struct CharTraits<char> {
-  template <typename T>
-  static int FormatFloat(char *buffer, std::size_t size,
-      const char *format, unsigned width, int precision, T value) {
+template <typename T>
+int fmt::internal::CharTraits<char>::FormatFloat(
+    char *buffer, std::size_t size, const char *format,
+    unsigned width, int precision, T value) {
   if (width == 0) {
     return precision < 0 ?
         FMT_SNPRINTF(buffer, size, format, value) :
@@ -90,14 +87,12 @@ struct CharTraits<char> {
   return precision < 0 ?
       FMT_SNPRINTF(buffer, size, format, width, value) :
       FMT_SNPRINTF(buffer, size, format, width, precision, value);
-  }
-};
+}
 
-template <>
-struct CharTraits<wchar_t> {
-  template <typename T>
-  static int FormatFloat(wchar_t *buffer, std::size_t size,
-      const wchar_t *format, unsigned width, int precision, T value) {
+template <typename T>
+int fmt::internal::CharTraits<wchar_t>::FormatFloat(
+    wchar_t *buffer, std::size_t size, const wchar_t *format,
+    unsigned width, int precision, T value) {
   if (width == 0) {
     return precision < 0 ?
         swprintf(buffer, size, format, value) :
@@ -106,8 +101,6 @@ struct CharTraits<wchar_t> {
   return precision < 0 ?
       swprintf(buffer, size, format, width, value) :
       swprintf(buffer, size, format, width, precision, value);
-  }
-};
 }
 
 const char fmt::internal::DIGITS[] =
@@ -310,7 +303,7 @@ void fmt::BasicWriter<Char>::FormatDouble(
   for (;;) {
     std::size_t size = buffer_.capacity() - offset;
     Char *start = &buffer_[offset];
-    int n = CharTraits<Char>::FormatFloat(
+    int n = internal::CharTraits<Char>::FormatFloat(
         start, size, format, width_for_sprintf, precision, value);
     if (n >= 0 && offset + n < buffer_.capacity()) {
       if (sign) {

format.h

@@ -148,6 +148,27 @@ void Array<T, SIZE>::append(const T *begin, const T *end) {
   size_ += num_elements;
 }
 
+template <typename Char>
+struct CharTraits;
+
+template <>
+struct CharTraits<char> {
+  typedef wchar_t UnsupportedType;
+
+  template <typename T>
+  static int FormatFloat(char *buffer, std::size_t size,
+      const char *format, unsigned width, int precision, T value);
+};
+
+template <>
+struct CharTraits<wchar_t> {
+  typedef char UnsupportedType;
+
+  template <typename T>
+  static int FormatFloat(wchar_t *buffer, std::size_t size,
+      const wchar_t *format, unsigned width, int precision, T value);
+};
+
 // Information about an integer type.
 // IntTraits is not specialized for integer types smaller than int,
 // since these are promoted to int.
@@ -501,6 +522,12 @@ class BasicWriter {
   CharPtr FormatString(const StringChar *s,
       std::size_t size, const FormatSpec &spec);
 
+  // This method is private to disallow writing a wide string to a
+  // char stream and vice versa. If you want to print a wide string
+  // as a pointer as std::ostream does, cast it to const void*.
+  // Do not implement!
+  void operator<<(const typename internal::CharTraits<Char>::UnsupportedType *);
+
  public:
   /**
     Returns the number of characters written to the output buffer.
@@ -596,13 +623,13 @@ class BasicWriter {
     return *this;
   }
 
-  BasicWriter &operator<<(Char value) {
+  BasicWriter &operator<<(char value) {
     *GrowBuffer(1) = value;
     return *this;
   }
 
-  BasicWriter &operator<<(const fmt::StringRef value) {
-    const char *str = value.c_str();
+  BasicWriter &operator<<(const fmt::BasicStringRef<Char> value) {
+    const Char *str = value.c_str();
     std::size_t size = value.size();
     std::copy(str, str + size, GrowBuffer(size));
     return *this;

format_test.cc

@@ -89,22 +89,50 @@ using fmt::pad;
   FORMAT_TEST_THROW_(statement, expected_exception, expected_message, \
       GTEST_NONFATAL_FAILURE_)
 
-struct WriteChecker {
-  template <typename T>
-  ::testing::AssertionResult operator()(const char *, const T &value) const {
-    std::ostringstream os;
+// Checks if writing value to BasicWriter<Char> produces the same result
+// as writing it to std::basic_ostringstream<Char>.
+template <typename Char, typename T>
+static ::testing::AssertionResult CheckWrite(const T &value, const char *type) {
+  std::basic_ostringstream<Char> os;
   os << value;
-    std::string expected = os.str(), actual = str(Writer() << value);
+  std::basic_string<Char> expected = os.str();
+  std::basic_string<Char> actual = str(BasicWriter<Char>() << value);
   if (expected == actual)
     return ::testing::AssertionSuccess();
   return ::testing::AssertionFailure()
-        << "Value of: str(Writer() << value)\n"
+      << "Value of: str(Writer<" << type << ">() << value)\n"
       << "  Actual: " << actual << "\n"
       << "Expected: " << expected << "\n";
+}
+
+struct AnyWriteChecker {
+  template <typename T>
+  ::testing::AssertionResult operator()(const char *, const T &value) const {
+    ::testing::AssertionResult result = CheckWrite<char>(value, "char");
+    return result ? CheckWrite<wchar_t>(value, "wchar_t") : result;
   }
 };
 
-#define CHECK_WRITE(value) ASSERT_PRED_FORMAT1(WriteChecker(), value)
+struct CharWriteChecker {
+  template <typename T>
+  ::testing::AssertionResult operator()(const char *, const T &value) const {
+    return CheckWrite<char>(value, "char");
+  }
+};
+
+struct WCharWriteChecker {
+  template <typename T>
+  ::testing::AssertionResult operator()(const char *, const T &value) const {
+    return CheckWrite<wchar_t>(value, "char");
+  }
+};
+
+// Checks if writing value to BasicWriter produces the same result
+// as writing it to std::ostringstream both for char and wchar_t.
+#define CHECK_WRITE(value) ASSERT_PRED_FORMAT1(AnyWriteChecker(), value)
+
+#define CHECK_WRITE_CHAR(value) ASSERT_PRED_FORMAT1(CharWriteChecker(), value)
+#define CHECK_WRITE_WCHAR(value) ASSERT_PRED_FORMAT1(WCharWriteChecker(), value)
 
 // Increment a number in a string.
 void Increment(char *s) {
@@ -279,8 +307,20 @@ TEST(WriterTest, WriteDouble) {
   CHECK_WRITE(4.2l);
 }
 
+TEST(WriterTest, WriteChar) {
+  CHECK_WRITE('a');
+}
+
 TEST(WriterTest, WriteString) {
-  CHECK_WRITE("abc");
+  CHECK_WRITE_CHAR("abc");
+  // The following line shouldn't compile:
+  //fmt::Writer() << L"abc";
+}
+
+TEST(WriterTest, WriteWideString) {
+  CHECK_WRITE_WCHAR(L"abc");
+  // The following line shouldn't compile:
+  //fmt::WWriter() << "abc";
 }
 
 TEST(WriterTest, oct) {