Add detection of wostream operator<< (#650)

include/fmt/core.h

@@ -413,7 +413,7 @@ FMT_CONSTEXPR bool is_arithmetic(type t) {
   return t > internal::none_type && t <= internal::last_numeric_type;
 }
 
-template <typename T, bool ENABLE = true>
+template <typename T, typename Char, bool ENABLE = true>
 struct convert_to_int {
   enum {
    value = !std::is_arithmetic<T>::value && std::is_convertible<T, int>::value
@@ -421,8 +421,8 @@ struct convert_to_int {
 };
 
 #define FMT_DISABLE_CONVERSION_TO_INT(Type) \
-  template <> \
-  struct convert_to_int<Type> { enum { value = 0 }; }
+  template <typename Char> \
+  struct convert_to_int<Type, Char> { enum { value = 0 }; }
 
 // Silence warnings about convering float to int.
 FMT_DISABLE_CONVERSION_TO_INT(float);
@@ -589,13 +589,13 @@ void make_value(const T *p) {
 
 template <typename C, typename T>
 inline typename std::enable_if<
-    convert_to_int<T>::value && std::is_enum<T>::value,
+    convert_to_int<T, typename C::char_type>::value && std::is_enum<T>::value,
     typed_value<C, int_type>>::type
   make_value(const T &val) { return static_cast<int>(val); }
 
 template <typename C, typename T>
-inline typename std::enable_if<
-    !convert_to_int<T>::value, typed_value<C, custom_type>>::type
+inline typename std::enable_if<!convert_to_int<
+    T, typename C::char_type>::value, typed_value<C, custom_type>>::type
   make_value(const T &val) { return val; }
 
 template <typename C, typename T>

include/fmt/ostream.h

@@ -46,21 +46,22 @@ class FormatBuf : public std::basic_streambuf<Char> {
   }
 };
 
-struct test_stream : std::ostream {
+template <typename Char>
+struct test_stream : std::basic_ostream<Char> {
  private:
   struct null;
-  // Hide all operator<< from std::ostream.
+  // Hide all operator<< from std::basic_ostream<Char>.
   void operator<<(null);
 };
 
 // Disable conversion to int if T has an overloaded operator<< which is a free
 // function (not a member of std::ostream).
-template <typename T>
-class convert_to_int<T, true> {
+template <typename T, typename Char>
+class convert_to_int<T, Char, true> {
  private:
   template <typename U>
   static decltype(
-    std::declval<test_stream&>() << std::declval<U>(), std::true_type())
+    std::declval<test_stream<Char>&>() << std::declval<U>(), std::true_type())
       test(int);
 
   template <typename>

test/ostream-test.cc

@@ -53,7 +53,7 @@ std::ostream &operator<<(std::ostream &os, TestEnum) {
 enum TestEnum2 {A};
 
 TEST(OStreamTest, Enum) {
-  EXPECT_FALSE(fmt::internal::convert_to_int<TestEnum>::value);
+  EXPECT_FALSE((fmt::internal::convert_to_int<TestEnum, char>::value));
   EXPECT_EQ("TestEnum", fmt::format("{}", TestEnum()));
   EXPECT_EQ("0", fmt::format("{}", A));
 }

test/util-test.cc

@@ -843,15 +843,15 @@ TEST(UtilTest, ReportWindowsError) {
 enum TestEnum2 {};
 
 TEST(UtilTest, ConvertToInt) {
-  EXPECT_FALSE(fmt::internal::convert_to_int<char>::value);
-  EXPECT_FALSE(fmt::internal::convert_to_int<const char *>::value);
-  EXPECT_TRUE(fmt::internal::convert_to_int<TestEnum2>::value);
+  EXPECT_FALSE((fmt::internal::convert_to_int<char, char>::value));
+  EXPECT_FALSE((fmt::internal::convert_to_int<const char *, char>::value));
+  EXPECT_TRUE((fmt::internal::convert_to_int<TestEnum2, char>::value));
 }
 
 #if FMT_USE_ENUM_BASE
 enum TestEnum : char {TestValue};
 TEST(UtilTest, IsEnumConvertibleToInt) {
-  EXPECT_TRUE(fmt::internal::convert_to_int<TestEnum>::value);
+  EXPECT_TRUE((fmt::internal::convert_to_int<TestEnum, char>::value));
 }
 #endif