Replace using of internal fmt lib API with public API

The commit cd5808eb introduced a union as a storage for the format
argument passed to the internal API fmt::detail::make_arg. This was done
to solve the issue that the internal API no longer accepted temporary
variables.

However, it's generally better to avoid using internal APIs, as they are
more likely to have breaking changes in the future. Instead, we can use
the public API fmt::dynamic_format_arg_store to dynamically build the
argument list. This API accepts temporary variables, and its behavior is
more stable than the internal API. `libfmt.cmake` is updated to reflect
the change as well.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.

cmake/libfmt.cmake

@@ -30,12 +30,13 @@ MACRO (CHECK_LIBFMT)
     CHECK_CXX_SOURCE_RUNS(
     "#define FMT_STATIC_THOUSANDS_SEPARATOR ','
      #define FMT_HEADER_ONLY 1
-     #include <fmt/format-inl.h>
+     #include <fmt/args.h>
      int main() {
+       using ArgStore= fmt::dynamic_format_arg_store<fmt::format_context>;
+       ArgStore arg_store;
        int answer= 4321;
-       fmt::format_args::format_arg arg=
-         fmt::detail::make_arg<fmt::format_context>(answer);
-       return fmt::vformat(\"{:L}\", fmt::format_args(&arg, 1)).compare(\"4,321\");
+       arg_store.push_back(answer);
+       return fmt::vformat(\"{:L}\", arg_store).compare(\"4,321\");
      }" HAVE_SYSTEM_LIBFMT)
     SET(CMAKE_REQUIRED_INCLUDES)
   ENDIF()

sql/item_strfunc.cc

@@ -58,7 +58,7 @@ C_MODE_END
 /* fmtlib include (https://fmt.dev/). */
 #define FMT_STATIC_THOUSANDS_SEPARATOR ','
 #define FMT_HEADER_ONLY 1
-#include "fmt/format-inl.h"
+#include "fmt/args.h"
 
 size_t username_char_length= USERNAME_CHAR_LENGTH;
 
@@ -1403,75 +1403,42 @@ namespace fmt {
 */
 String *Item_func_sformat::val_str(String *res)
 {
-  /*
-    A union that stores a numeric format arg value.
-    fmt::detail::make_arg does not accept temporaries, so all of its numeric
-    args are temporarily stored in the fmt_args array.
-    See: https://github.com/fmtlib/fmt/issues/3596
-  */
-  union Format_arg_store {
-    longlong val_int;
-    float    val_float;
-    double   val_double;
-  };
-
   DBUG_ASSERT(fixed());
-  using                         ctx=      fmt::format_context;
-  String                       *fmt_arg=  NULL;
-  String                       *parg=     NULL;
-  fmt::format_args::format_arg *vargs=    NULL;
-  Format_arg_store             *fmt_args= NULL;
+  using         ArgStore= fmt::dynamic_format_arg_store<fmt::format_context>;
+  String       *fmt_arg=  NULL;
+  String       *parg=     NULL;
+  ArgStore      arg_store;
 
   null_value= true;
   if (!(fmt_arg= args[0]->val_str(res)))
     return NULL;
 
-  if (!(vargs= new fmt::format_args::format_arg[arg_count - 1]))
-    return NULL;
-
-  if (!(fmt_args= new Format_arg_store[arg_count - 1]))
-  {
-    delete [] vargs;
-    return NULL;
-  }
-
   /* Creates the array of arguments for vformat */
   for (uint carg= 1; carg < arg_count; carg++)
   {
     switch (args[carg]->result_type())
     {
     case INT_RESULT:
-      fmt_args[carg-1].val_int= args[carg]->val_int();
-      vargs[carg-1]= fmt::detail::make_arg<ctx>(fmt_args[carg-1].val_int);
+      arg_store.push_back(args[carg]->val_int());
       break;
     case DECIMAL_RESULT: // TODO
     case REAL_RESULT:
       if (args[carg]->field_type() == MYSQL_TYPE_FLOAT)
-      {
-        fmt_args[carg-1].val_float= (float)args[carg]->val_real();
-        vargs[carg-1]= fmt::detail::make_arg<ctx>(fmt_args[carg-1].val_float);
-      }
+        arg_store.push_back((float)args[carg]->val_real());
       else
-      {
-        fmt_args[carg-1].val_double= args[carg]->val_real();
-        vargs[carg-1]= fmt::detail::make_arg<ctx>(fmt_args[carg-1].val_double);
-      }
+        arg_store.push_back(args[carg]->val_real());
       break;
     case STRING_RESULT:
       if (!(parg= args[carg]->val_str(&val_arg[carg-1])))
       {
-        delete [] vargs;
-        delete [] fmt_args;
         return NULL;
       }
-      vargs[carg-1]= fmt::detail::make_arg<ctx>(*parg);
+      arg_store.push_back(*parg);
       break;
     case TIME_RESULT: // TODO
     case ROW_RESULT: // TODO
     default:
       DBUG_ASSERT(0);
-      delete [] vargs;
-      delete [] fmt_args;
       return NULL;
     }
   }
@@ -1480,8 +1447,7 @@ String *Item_func_sformat::val_str(String *res)
   /* Create the string output  */
   try
   {
-    auto text = fmt::vformat(fmt_arg->c_ptr_safe(),
-                             fmt::format_args(vargs, arg_count-1));
+    auto text = fmt::vformat(fmt_arg->c_ptr_safe(), arg_store);
     res->length(0);
     res->set_charset(collation.collation);
     res->append(text.c_str(), text.size(), fmt_arg->charset());
@@ -1494,8 +1460,6 @@ String *Item_func_sformat::val_str(String *res)
                         ER_THD(thd, WARN_SFORMAT_ERROR), ex.what());
     null_value= true;
   }
-  delete [] vargs;
-  delete [] fmt_args;
   return null_value ? NULL : res;
 }