rpc: fix compilation error caused by fmt::runtime()

Depending on the 'SEASTAR_LOGGER_COMPILE_TIME_FMT' the declaration
of 'logger::log()' function may use either 'fmt::format_string' or
'const char*' format parameter.

In the case of the usage of 'const char*', there is a need to call
'fmt::runtime()' function to obtain format_string.

When the project was built on Ubuntu 22.04 using the steps described
in the tutorial, there was a compilation error related to passing
argument with type 'fmt::format_string<>' to 'fmt::runtime()' that
handles characters strings.

This change introduces contexpr if statement to remove the compilation
error and use 'fmt::runtime()' only when the function takes 'const char*'.

Signed-off-by: Patryk Wrobel <[email protected]>

include/seastar/rpc/rpc.hh

@@ -21,6 +21,7 @@
 
 #pragma once
 
+#include <type_traits>
 #include <unordered_map>
 #include <unordered_set>
 #include <list>
@@ -218,7 +219,13 @@ class logger {
  // i.e. when the user still only defines a level-less logger.
  } else if (_logger && level <= log_level::info) {
  fmt::memory_buffer out;
- fmt::format_to(fmt::appender(out), fmt::runtime(fmt), std::forward<Args>(args)...);
+
+ if constexpr (std::is_same_v<const char*, decltype(fmt)>) {
+ fmt::format_to(fmt::appender(out), fmt::runtime(fmt), std::forward<Args>(args)...);
+ } else {
+ fmt::format_to(fmt::appender(out), fmt, std::forward<Args>(args)...);
+ }
+
  _logger(sstring{out.data(), out.size()});
  }
  }