Expose GetSlots and CreateSlots instead of the slots itself

to avoid dual package hazards

Author: tars0x9752

lib/slots.ts

@@ -150,19 +150,30 @@ interface SlotsToTypes {
 
 type SlotKey = keyof SlotsToTypes;
 
-const globalTemporalSlotMapKey = Symbol.for('@@Temporal__slots__private_do_not_use');
+const globalSlots = new WeakMap();
 
-(globalThis as any)[globalTemporalSlotMapKey] ||= new WeakMap();
+function _GetSlots(container: Slots[SlotKey]['usedBy']) {
+  return globalSlots.get(container);
+}
 
-const slots = (globalThis as any)[globalTemporalSlotMapKey];
+const GetSlotsSymbol = Symbol.for('@@Temporal__GetSlots');
 
-export function CreateSlots(container: AnyTemporalType): void {
-  slots.set(container, Object.create(null));
-}
+// expose GetSlots to avoid dual package hazards
+(globalThis as any)[GetSlotsSymbol] ||= _GetSlots;
+
+const GetSlots = (globalThis as any)[GetSlotsSymbol];
 
-function GetSlots<T extends AnyTemporalType>(container: T) {
-  return slots.get(container);
+function _CreateSlots(container: Slots[SlotKey]['usedBy']): void {
+  globalSlots.set(container, Object.create(null));
 }
+
+const CreateSlotsSymbol = Symbol.for('@@Temporal__CreateSlots');
+
+// expose CreateSlots to avoid dual package hazards
+(globalThis as any)[CreateSlotsSymbol] ||= _CreateSlots;
+
+export const CreateSlots = (globalThis as any)[CreateSlotsSymbol];
+
 // TODO: is there a better way than 9 overloads to make HasSlot into a type
 // guard that takes a variable number of parameters?
 export function HasSlot<ID1 extends SlotKey>(container: unknown, id1: ID1): container is Slots[ID1]['usedBy'];
@@ -292,5 +303,11 @@ export function SetSlot<KeyT extends SlotKey>(
   id: KeyT,
   value: Slots[KeyT]['value']
 ): void {
-  GetSlots(container)[id] = value;
+  const slot = GetSlots(container);
+
+  if (id in slot) {
+    throw new TypeError(`${id} already has set`);
+  }
+
+  slot[id] = value;
 }