fix fmt can't format enum

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,