Fix handling of types convertible to int

fmt/ostream.h

@@ -52,8 +52,10 @@ Yes &convert(std::ostream &);
 
 struct DummyStream : std::ostream {
   DummyStream();  // Suppress a bogus warning in MSVC.
+
   // Hide all operator<< overloads from std::ostream.
-  void operator<<(Null<>);
+  template <typename T>
+  typename EnableIf<sizeof(T) == 0>::type operator<<(const T &);
 };
 
 No &operator<<(std::ostream &, int);

test/ostream-test.cc

@@ -172,3 +172,18 @@ TEST(OStreamTest, WriteToOStreamMaxSize) {
   } while (size != 0);
   fmt::internal::write(os, w);
 }
+
+struct ConvertibleToInt {
+  template <typename ValueType>
+  operator ValueType() const {
+    return 0;
+  }
+
+  friend std::ostream &operator<<(std::ostream &o, ConvertibleToInt) {
+    return o << "foo";
+  }
+};
+
+TEST(FormatTest, FormatConvertibleToInt) {
+  EXPECT_EQ("foo", fmt::format("{}", ConvertibleToInt()));
+}