fix fmt can't format enum

CMakeLists.txt

@@ -124,6 +124,7 @@ if(SLED_BUILD_TESTS)
 		src/any_test.cc
         src/filesystem/path_test.cc
 		src/futures/future_test.cc
+        src/log/fmt_test.cc
 		# src/profiling/profiling_test.cc
         src/strings/base64_test.cc
         src/synchronization/sequence_checker_test.cc

include/sled/log/log.h

@@ -7,6 +7,7 @@
 #ifndef SLED_LOG_LOG_H
 #define SLED_LOG_LOG_H
 #pragma once
+
 #include "sled/system/location.h"
 #include <assert.h>
 #include <fmt/chrono.h>
@@ -19,6 +20,13 @@
 #include <fmt/std.h>
 #include <fmt/xchar.h>
 
+template<typename T, typename std::enable_if<std::is_enum<T>::value, int>::type = 0>
+auto
+format_as(const T &value) -> int
+{
+    return static_cast<int>(value);
+}
+
 namespace sled {
 enum class LogLevel {
     kTrace = 0,

src/log/fmt_test.cc

@@ -0,0 +1,24 @@
+#include <gtest/gtest.h>
+#include <sled/log/log.h>
+
+TEST(format, enum)
+{
+    enum EnumType {
+        kOne = 1,
+        kTwo = 2,
+        kThree = 3,
+    };
+
+    EXPECT_EQ(fmt::format("{}{}{}", kOne, kTwo, kThree), "123");
+}
+
+TEST(format, neg_enum)
+{
+    enum EnumType {
+        kOne = -1,
+        kTwo = -2,
+        kThree = -3,
+    };
+
+    EXPECT_EQ(fmt::format("{}{}{}", kOne, kTwo, kThree), "-1-2-3");
+}