Added generic typing to flatMap method.

Author: mad-briller

src/Illuminate/Collections/Enumerable.php

@@ -680,8 +680,11 @@ public function mapWithKeys(callable $callback);
     /**
      * Map a collection and flatten the result by a single level.
      *
-     * @param  callable(TValue, TKey): mixed  $callback
-     * @return static<int, mixed>
+     * @template TFlatMapKey of array-key
+     * @template TFlatMapValue
+     *
+     * @param  callable(TValue, TKey): (\Illuminate\Support\Collection<TFlatMapKey, TFlatMapValue>|array<TFlatMapKey, TFlatMapValue>)  $callback
+     * @return static<TFlatMapKey, TFlatMapValue>
      */
     public function flatMap(callable $callback);
 

src/Illuminate/Collections/Traits/EnumeratesValues.php

@@ -384,8 +384,11 @@ public function mapToGroups(callable $callback)
     /**
      * Map a collection and flatten the result by a single level.
      *
-     * @param  callable(TValue, TKey): mixed  $callback
-     * @return static<int, mixed>
+     * @template TFlatMapKey of array-key
+     * @template TFlatMapValue
+     *
+     * @param  callable(TValue, TKey): (\Illuminate\Support\Collection<TFlatMapKey, TFlatMapValue>|array<TFlatMapKey, TFlatMapValue>)  $callback
+     * @return static<TFlatMapKey, TFlatMapValue>
      */
     public function flatMap(callable $callback)
     {

types/Support/Collection.php

@@ -584,12 +584,12 @@ function ($collection, $count) {
         return ['string' => 1];
     }));
 
-assertType('Illuminate\Support\Collection<int, mixed>', $collection::make(['string'])
+assertType('Illuminate\Support\Collection<int, string>', $collection::make(['string'])
     ->flatMap(function ($string, $int) {
         assertType('string', $string);
         assertType('int', $int);
 
-        return 1;
+        return [0 => 'string'];
     }));
 
 assertType('Illuminate\Support\Collection<int, User>', $collection->mapInto(User::class));

types/Support/LazyCollection.php

@@ -480,12 +480,12 @@
         return ['string' => 1];
     }));
 
-assertType('Illuminate\Support\LazyCollection<int, mixed>', $collection::make(['string'])
+assertType('Illuminate\Support\LazyCollection<int, string>', $collection::make(['string'])
     ->flatMap(function ($string, $int) {
         assertType('string', $string);
         assertType('int', $int);
 
-        return 1;
+        return [0 => 'string'];
     }));
 
 assertType('Illuminate\Support\LazyCollection<int, User>', $collection->mapInto(User::class));