Fix type check of unsupported language codes in Stripe checkout session

apps/api/src/app/subscription/subscription.service.ts

@@ -23,6 +23,8 @@ import { addMilliseconds, isBefore } from 'date-fns';
 import ms, { StringValue } from 'ms';
 import Stripe from 'stripe';
 
+import { StringLiteral } from './types/string-literal.type';
+
 @Injectable()
 export class SubscriptionService {
   private readonly logger = new Logger(SubscriptionService.name);
@@ -268,7 +270,7 @@ export class SubscriptionService {
     const unsupportedLanguageCodes: Record<
       Exclude<
         (typeof SUPPORTED_LANGUAGE_CODES)[number],
-        Stripe.Checkout.SessionCreateParams.Locale
+        StringLiteral<Stripe.Checkout.SessionCreateParams.Locale>
       >,
       true
     > = {

apps/api/src/app/subscription/types/string-literal.type.ts

@@ -0,0 +1,7 @@
+// Removes the open string type (e.g. OtherString of Stripe), which matches
+// every string, and keeps only the string literals
+export type StringLiteral<T> = T extends string
+  ? string extends T
+    ? never
+    : T
+  : never;