Fix handling of streamable and convertible to string types

include/fmt/core.h

@@ -905,6 +905,8 @@ template <typename Context> struct arg_mapper {
   }
   template <typename T,
             FMT_ENABLE_IF(!is_string<T>::value && !is_char<T>::value &&
+                          !std::is_constructible<basic_string_view<char_type>,
+                                                 T>::value &&
                           (has_formatter<T, Context>::value ||
                            has_fallback_formatter<T, Context>::value))>
   FMT_CONSTEXPR const T& map(const T& val) {

test/core-test.cc

@@ -635,19 +635,6 @@ TEST(FormatterTest, FormatExplicitlyConvertibleToWStringView) {
   EXPECT_EQ(L"foo",
             fmt::format(L"{}", explicitly_convertible_to_wstring_view()));
 }
-
-struct explicitly_convertible_to_string_like {
-  template <typename String,
-            typename = typename std::enable_if<std::is_constructible<
-                String, const char*, std::size_t>::value>::type>
-  explicit operator String() const {
-    return String("foo", 3u);
-  }
-};
-
-TEST(FormatterTest, FormatExplicitlyConvertibleToStringLike) {
-  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_like()));
-}
 #endif
 
 struct disabled_rvalue_conversion {

test/ostream-test.cc

@@ -243,8 +243,7 @@ TEST(FormatTest, UDL) {
 }
 #endif
 
-template <typename T>
-struct convertible {
+template <typename T> struct convertible {
   T value;
   explicit convertible(const T& val) : value(val) {}
   operator T() const { return value; }
@@ -255,3 +254,25 @@ TEST(OStreamTest, DisableBuiltinOStreamOperators) {
   EXPECT_EQ(L"42", fmt::format(L"{:d}", convertible<unsigned short>(42)));
   EXPECT_EQ("foo", fmt::format("{}", convertible<const char*>("foo")));
 }
+
+struct explicitly_convertible_to_string_like {
+  template <typename String,
+            typename = typename std::enable_if<std::is_constructible<
+                String, const char*, std::size_t>::value>::type>
+  explicit operator String() const {
+    return String("foo", 3u);
+  }
+};
+
+TEST(FormatterTest, FormatExplicitlyConvertibleToStringLike) {
+  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_like()));
+}
+
+std::ostream& operator<<(std::ostream& os,
+                         explicitly_convertible_to_string_like) {
+  return os << "bar";
+}
+
+TEST(FormatterTest, FormatExplicitlyConvertibleToStringLikeIgnoreInserter) {
+  EXPECT_EQ("foo", fmt::format("{}", explicitly_convertible_to_string_like()));
+}
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