Fix MSVC build, take 2

test/format-test.cc

@@ -1423,13 +1423,19 @@ class mock_arg_formatter:
     EXPECT_CALL(*this, call(42));
   }
 
-  using base::operator();
-
-  iterator operator()(int value) {
+  template <typename T>
+  typename std::enable_if<std::is_integral<T>::value, iterator>::type
+      operator()(T value) {
     call(value);
     return base::operator()(value);
   }
 
+  template <typename T>
+  typename std::enable_if<!std::is_integral<T>::value, iterator>::type
+      operator()(T value) {
+    return base::operator()(value);
+  }
+
   iterator operator()(fmt::basic_format_arg<fmt::format_context>::handle) {
     return base::operator()(fmt::monostate());
   }