✨ Allow fmt formatter to format runtime enum values

Problem:
- Runtime enumerations cannot in general be formatted by the fmt formatter, but
  work fine with the binary formatter.

Solution:
- Decay enumeration values to their `underlying_type` iff no `format_as`
  function is provided for them.

Author: elbeno

include/log/fmt/logger.hpp

@@ -24,6 +24,14 @@ namespace fmt_detail {
 using namespace std::string_view_literals;
 constexpr std::array level_text{"MAX"sv,  "FATAL"sv, "ERROR"sv, "WARN"sv,
                                 "INFO"sv, "USER1"sv, "USER2"sv, "TRACE"sv};
+
+template <typename T> auto decay_enum_value(T const &t) -> decltype(auto) {
+    if constexpr (requires { format_as(t); } or not std::is_enum_v<T>) {
+        return (t);
+    } else {
+        return stdx::to_underlying(t);
+    }
+}
 } // namespace fmt_detail
 
 template <logging::level L>
@@ -55,7 +63,8 @@ template <typename TDestinations> struct log_handler {
                 constexpr auto fmtstr =
                     std::string_view{decltype(fr.str)::value};
                 fr.args.apply([&](auto const &...args) {
-                    ::fmt::format_to(out, fmtstr, args...);
+                    ::fmt::format_to(out, fmtstr,
+                                     fmt_detail::decay_enum_value(args)...);
                 });
                 *out = '\n';
             },

test/log/fmt_logger.cpp

@@ -53,13 +53,35 @@ TEST_CASE("logging doesn't use dynamic memory", "[fmt_logger]") {
 }
 
 TEST_CASE("logging behavior can be properly overridden", "[fmt_logger]") {
-    buffer.reserve(100);
     buffer.clear();
     log_test_override();
     CAPTURE(buffer);
     CHECK(buffer.substr(buffer.size() - std::size("Hello")) == "Hello\n");
 }
 
+namespace detail {
+enum struct E1 { A, B, C };
+enum struct E2 { A, B, C };
+[[maybe_unused]] static auto format_as(E2) { return 42; }
+} // namespace detail
+
+TEST_CASE("fmt logger can deal with runtime enum values", "[fmt_logger]") {
+    buffer.clear();
+    auto x = detail::E1::A;
+    CIB_INFO("Hello {}", x);
+    CAPTURE(buffer);
+    CHECK(buffer.substr(buffer.size() - std::size("Hello 0")) == "Hello 0\n");
+}
+
+TEST_CASE("fmt logger can deal with formattable runtime enum values",
+          "[fmt_logger]") {
+    buffer.clear();
+    auto x = detail::E2::A;
+    CIB_INFO("Hello {}", x);
+    CAPTURE(buffer);
+    CHECK(buffer.substr(buffer.size() - std::size("Hello 42")) == "Hello 42\n");
+}
+
 TEST_CASE("log levels are properly represented", "[fmt_logger]") {
     {
         std::string level{};