Add specialization test

include/fmt/ostream.h

@@ -112,9 +112,11 @@ struct fallback_formatter<
 
 // Disable conversion to int if T has an overloaded operator<< which is a free
 // function (not a member of std::ostream).
-template <typename T, typename Char> struct convert_to_int<T, Char, void> {
-  static const bool value = convert_to_int<T, Char, int>::value &&
-                            !internal::is_streamable<T, Char>::value;
+template <typename T, typename Char>
+struct convert_to_int<
+    T, Char,
+    typename std::enable_if<internal::is_streamable<T, Char>::value>::type> {
+  static const bool value = false;
 };
 
 template <typename Char>

test/ostream-test.cc

@@ -6,6 +6,21 @@
 // For the license information refer to format.h.
 
 #define FMT_STRING_ALIAS 1
+#include "fmt/format.h"
+
+struct test {};
+
+// Test that there is no issues with specializations when fmt/ostream.h is
+// included after fmt/format.h.
+namespace fmt {
+template <> struct formatter<test> : formatter<int> {
+  template <typename FormatContext>
+  typename FormatContext::iterator format(const test&, FormatContext& ctx) {
+    return formatter<int>::format(42, ctx);
+  }
+};
+}  // namespace fmt
+
 #include "fmt/ostream.h"
 
 #include <sstream>