T command output the current active extruder when no new tool index passed

Marlin/src/gcode/control/T.cpp

@@ -41,13 +41,21 @@
  *   S1           Don't move the tool in XY after change
  *
  * For PRUSA_MMU2(S) and EXTENDABLE_EMU_MMU2(S)
- *   T[n] Gcode to extrude at least 38.10 mm at feedrate 19.02 mm/s must follow immediately to load to extruder wheels.
- *   T?   Gcode to extrude shouldn't have to follow. Load to extruder wheels is done automatically.
+ *   T[n] G-code to extrude at least 38.10 mm at feedrate 19.02 mm/s must follow immediately to load to extruder wheels.
+ *   T?   G-code to extrude shouldn't have to follow. Load to extruder wheels is done automatically.
  *   Tx   Same as T?, but nozzle doesn't have to be preheated. Tc requires a preheated nozzle to finish filament load.
  *   Tc   Load to nozzle after filament was prepared by Tc and nozzle is already heated.
  */
 void GcodeSuite::T(const int8_t tool_index) {
 
+  #if HAS_MULTI_EXTRUDER
+    // For 'T' with no parameter report the current tool.
+    if (parser.string_arg && *parser.string_arg == '*') {
+      SERIAL_ECHOLNPGM(STR_ACTIVE_EXTRUDER, active_extruder);
+      return;
+    }
+  #endif
+
   DEBUG_SECTION(log_T, "T", DEBUGGING(LEVELING));
   if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPGM("...(", tool_index, ")");
 

Marlin/src/gcode/parser.cpp

@@ -189,7 +189,13 @@ void GCodeParser::parse(char *p) {
       #endif
 
       // Bail if there's no command code number
-      if (!TERN(SIGNED_CODENUM, NUMERIC_SIGNED(*p), NUMERIC(*p))) return;
+      if (!TERN(SIGNED_CODENUM, NUMERIC_SIGNED(*p), NUMERIC(*p))) {
+        if (TERN0(HAS_MULTI_EXTRUDER, letter == 'T')) {
+          p[0] = '*'; p[1] = '\0'; string_arg = p; // Convert 'T' alone into 'T*'
+          command_letter = letter;
+        }
+        return;
+      }
 
       // Save the command letter at this point
       // A '?' signifies an unknown command