From 95489ddef0bd79df380d04c4ef601e3a7f1bb9ad Mon Sep 17 00:00:00 2001
From: Pedro Durek <pedro.durek@shopify.com>
Date: Sat, 12 Nov 2022 09:35:23 -0700
Subject: [PATCH] Change `DefaultTFuncReturn` to return `null` if `returnNull`
 typeOption is true (#1865)

* Change DefaultTFuncReturn to return null if returnNull typeOption is true

* Fix tests
---
 index.d.ts                                | 2 +-
 test/typescript/custom-types/i18next.d.ts | 1 +
 test/typescript/custom-types/t.test.ts    | 7 +++++++
 test/typescript/t.test.ts                 | 7 -------
 4 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/index.d.ts b/index.d.ts
index 01fb68b30..d28c6fc9f 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -860,7 +860,7 @@ type NormalizeMultiReturn<T, V> = V extends `${infer N}:${infer R}`
     : never
   : never;
 
-export type DefaultTFuncReturn = string | undefined | null;
+export type DefaultTFuncReturn = string | (TypeOptions['returnNull'] extends true ? null : never);
 
 export type DefaultTFuncReturnWithObject = DefaultTFuncReturn | object | Array<string | object>;
 
diff --git a/test/typescript/custom-types/i18next.d.ts b/test/typescript/custom-types/i18next.d.ts
index 1a5b39149..562adbfb3 100644
--- a/test/typescript/custom-types/i18next.d.ts
+++ b/test/typescript/custom-types/i18next.d.ts
@@ -3,6 +3,7 @@ import 'i18next';
 declare module 'i18next' {
   interface CustomTypeOptions {
     defaultNS: 'custom';
+    returnNull: false;
     resources: {
       custom: {
         foo: 'foo';
diff --git a/test/typescript/custom-types/t.test.ts b/test/typescript/custom-types/t.test.ts
index b31f16acd..634e2e33e 100644
--- a/test/typescript/custom-types/t.test.ts
+++ b/test/typescript/custom-types/t.test.ts
@@ -45,3 +45,10 @@ function i18nextTPluralsUsage() {
   i18next.t('plurals:foo', { count: 1 });
   i18next.t('plurals:foo_many', { count: 10 });
 }
+
+// @ts-expect-error
+function returnNullWithFalseValue(t: TFunction<string>) {
+  function fn(value: null) {}
+  // @ts-expect-error
+  fn(t('foo'));
+}
diff --git a/test/typescript/t.test.ts b/test/typescript/t.test.ts
index 0bfc3df9c..343c876f0 100644
--- a/test/typescript/t.test.ts
+++ b/test/typescript/t.test.ts
@@ -43,13 +43,6 @@ function callsMethodWithOptionalNullArg(t: TFunction) {
   displayHint(t('friend'));
 }
 
-function callsMethodWithOptionalArg(t: TFunction) {
-  function displayHint(hint?: string) {
-    return String(hint);
-  }
-  displayHint(t('friend'));
-}
-
 function callsMethodWithRequiredNullArg(t: TFunction) {
   function displayHint(hint: string | null) {
     return String(hint);