Fix is_formattable for tuple-like types.

Author: jehelset

include/fmt/ranges.h

@@ -202,6 +202,24 @@ template <size_t N>
 using make_index_sequence = make_integer_sequence<size_t, N>;
 #endif
 
+template <typename T>
+using tuple_index_sequence = make_index_sequence<std::tuple_size_v<T>>;
+
+template <typename T, bool = is_tuple_like_<T>::value>
+struct is_tuple_formattable_ {
+  static constexpr const bool value = false;
+};
+template <typename T> struct is_tuple_formattable_<T, true> {
+  template <std::size_t... I>
+  static std::integral_constant<
+      bool, (fmt::is_formattable<std::tuple_element_t<I, T>>::value && ...)>
+      check(index_sequence<I...>);
+
+ public:
+  static constexpr const bool value =
+      decltype(check(tuple_index_sequence<T>{}))::value;
+};
+
 template <class Tuple, class F, size_t... Is>
 void for_each(index_sequence<Is...>, Tuple&& tup, F&& f) noexcept {
   using std::get;
@@ -283,8 +301,14 @@ template <typename T> struct is_tuple_like {
       detail::is_tuple_like_<T>::value && !detail::is_range_<T>::value;
 };
 
+template <typename T> struct is_tuple_formattable {
+  static constexpr const bool value = detail::is_tuple_formattable_<T>::value;
+};
+
 template <typename TupleT, typename Char>
-struct formatter<TupleT, Char, enable_if_t<fmt::is_tuple_like<TupleT>::value>> {
+struct formatter<TupleT, Char,
+                 enable_if_t<fmt::is_tuple_like<TupleT>::value &&
+                             fmt::is_tuple_formattable<TupleT>::value>> {
  private:
   // C++11 generic lambda for format().
   template <typename FormatContext> struct format_each {

test/ranges-test.cc

@@ -90,6 +90,18 @@ TEST(ranges_test, format_tuple) {
       std::tuple<int, float, std::string, char>(42, 1.5f, "this is tuple", 'i');
   EXPECT_EQ(fmt::format("{}", t), "(42, 1.5, \"this is tuple\", 'i')");
   EXPECT_EQ(fmt::format("{}", std::tuple<>()), "()");
+
+  enum class noformatenum{b};
+  struct noformatstruct{};
+  EXPECT_TRUE((fmt::is_formattable<std::tuple<>>::value));
+  EXPECT_FALSE((fmt::is_formattable<noformatenum>::value));
+  EXPECT_FALSE((fmt::is_formattable<noformatstruct>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::tuple<noformatenum>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::tuple<noformatstruct>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::tuple<noformatstruct,int>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::tuple<int,noformatenum>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::tuple<noformatstruct,noformatenum>>::value));
+  EXPECT_TRUE((fmt::is_formattable<std::tuple<int,float>>::value));
 }
 
 #ifdef FMT_RANGES_TEST_ENABLE_FORMAT_STRUCT

test/xchar-test.cc

@@ -315,10 +315,12 @@ TEST(xchar_test, ostream) {
 #endif
 }
 
+#ifdef FMT_XCHAR_TEST_ENABLE_FORMAT_MAP
 TEST(xchar_test, format_map) {
   auto m = std::map<std::wstring, int>{{L"one", 1}, {L"t\"wo", 2}};
   EXPECT_EQ(fmt::format(L"{}", m), L"{\"one\": 1, \"t\\\"wo\": 2}");
 }
+#endif
 
 TEST(xchar_test, escape_string) {
   using vec = std::vector<std::wstring>;