Add [[fallthrough]];.

src/action/action_board.cpp

@@ -271,7 +271,7 @@ void COrder_Board::Execute(CUnit &unit) /* override */
 				return;
 			}
 			this->State = 1;
-		// FALL THROUGH
+			[[fallthrough]];
 		default: { // Move to transporter
 			if (this->State <= State_MoveToTransporterMax) {
 				const int pathRet = MoveToTransporter(unit);

src/action/action_follow.cpp

@@ -269,7 +269,7 @@ void COrder_Follow::Execute(CUnit &unit) /* override */
 			this->goalPos = goal->tilePos;
 			this->State = State_TargetReached;
 		}
-		// FALL THROUGH
+			[[fallthrough]];
 		default:
 			break;
 	}

src/action/action_repair.cpp

@@ -243,9 +243,7 @@ void COrder_Repair::Execute(CUnit &unit) /* override */
 	Assert(this->ReparableTarget == this->GetGoal());
 
 	switch (this->State) {
-		case 0:
-			this->State = 1;
-		// FALL THROUGH
+		case 0: this->State = 1; [[fallthrough]];
 		case 1: { // Move near to target.
 			// FIXME: RESET FIRST!! Why? We move first and than check if
 			// something is in sight.

src/action/action_spellcast.cpp

@@ -351,7 +351,7 @@ void COrder_SpellCast::Execute(CUnit &unit) /* override */
 			// FIXME FIXME FIXME: Check if already in range and skip straight to 2(casting)
 			unit.ReCast = 0; // repeat spell on next pass? (defaults to `no')
 			this->State = 1;
-		// FALL THROUGH
+			[[fallthrough]];
 		case 1:                         // Move to the target.
 			if (spell.Range != INFINITE_RANGE) {
 				if (SpellMoveToTarget(unit) == true) {
@@ -364,7 +364,7 @@ void COrder_SpellCast::Execute(CUnit &unit) /* override */
 			} else {
 				this->State = 2;
 			}
-		// FALL THROUGH
+			[[fallthrough]];
 		case 2:                         // Cast spell on the target.
 			if (!spell.IsCasterOnly() || spell.ForceUseAnimation) {
 				AnimateActionSpellCast(unit, *this);

src/ui/botpanel.cpp

@@ -883,7 +883,7 @@ bool IsButtonAllowed(const CUnit &unit, const ButtonAction &buttonaction)
 			if (!EnableTrainingQueue && unit.CurrentAction() == UnitAction::Train) {
 				break;
 			}
-		// FALL THROUGH
+			[[fallthrough]];
 		case ButtonCmd::UpgradeTo:
 		case ButtonCmd::Research:
 		case ButtonCmd::Build:

src/ui/interface.cpp

@@ -660,7 +660,7 @@ static bool CommandKey(int key)
 			if (!(KeyModifiers & (ModifierAlt | ModifierControl))) {
 				break;
 			}
-		// FALL THROUGH
+			[[fallthrough]];
 		case SDLK_PERIOD: // ., ALT+I, CTRL+I: Find idle worker
 			UiFindIdleWorker();
 			break;
@@ -677,7 +677,7 @@ static bool CommandKey(int key)
 			if (!(KeyModifiers & (ModifierAlt | ModifierControl))) {
 				break;
 			}
-		// FALL THROUGH (CTRL+P, ALT+P)
+			[[fallthrough]]; // (CTRL+P, ALT+P)
 		case SDLK_PAUSE:
 			UiTogglePause();
 			break;
@@ -887,7 +887,7 @@ static void InputKey(int key)
 				NetworkSendChatMessage(chatMessage);
 			}
 		}
-	// FALL THROUGH
+			[[fallthrough]];
 		case SDLK_ESCAPE:
 			KeyState = EKeyState::Command;
 			UI.StatusLine.Clear();