diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 2c581fd1..a361c88b 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -1,12 +1,26 @@
 name: Test
 
-on: [push, pull_request]
+on:
+  pull_request:
+    branches: ["main", "master"]
+    paths:
+      - "src/**"
+      - "tests/**"
+      - "composer.json"
+      - ".github/workflows/test.yaml"
+  push:
+    branches: ["main", "master"]
+    paths:
+      - "src/**"
+      - "tests/**"
+      - "composer.json"
+      - ".github/workflows/test.yaml"
 
 jobs:
     test:
         name: PHP ${{ matrix.php-version }} (${{ matrix.experimental && 'experimental' || 'full support' }})
 
-        runs-on: ubuntu-18.04
+        runs-on: ubuntu-22.04
 
         strategy:
             fail-fast: false
@@ -17,15 +31,19 @@ jobs:
                     - 7.3
                     - 7.4
                     - 8.0
+                    - 8.1
+                    - 8.2
+                    - 8.3
+                    - 8.4
                 experimental: [false]
                 include:
-                    - php-version: 8.1
+                    - php-version: 8.5
                       experimental: true
 
         continue-on-error: ${{ matrix.experimental }}
         steps:
             - name: Checkout
-              uses: actions/checkout@v2
+              uses: actions/checkout@v5
 
             - name: Install PHP with extensions
               uses: shivammathur/setup-php@v2
@@ -35,7 +53,7 @@ jobs:
                   tools: composer:v2
 
             - name: Install Composer dependencies
-              uses: ramsey/composer-install@v1
+              uses: ramsey/composer-install@v3
               with:
                   composer-options: --prefer-dist
               continue-on-error: ${{ matrix.experimental }}
@@ -45,6 +63,7 @@ jobs:
                 composer require pcov/clobber
                 vendor/bin/pcov clobber
               continue-on-error: true
+              if: ${{ matrix.php-version == '7.1' }}
 
             - name: Run Tests
               run: composer tests
@@ -53,3 +72,5 @@ jobs:
             - name: Check coding style
               run: composer coding-style
               continue-on-error: ${{ matrix.experimental }}
+              env:
+                PHP_CS_FIXER_IGNORE_ENV: 1
diff --git a/src/Functional/CompareObjectHashOn.php b/src/Functional/CompareObjectHashOn.php
index c4194479..35e0ce04 100644
--- a/src/Functional/CompareObjectHashOn.php
+++ b/src/Functional/CompareObjectHashOn.php
@@ -14,11 +14,11 @@
  * Returns a comparison function that can be used with e.g. `usort()`
  *
  * @param callable $comparison A function that compares the two values. Pick e.g. strcmp() or strnatcasecmp()
- * @param callable $keyFunction A function that takes an argument and returns the value that should be compared
+ * @param callable|null $keyFunction A function that takes an argument and returns the value that should be compared
  * @return callable
  * @no-named-arguments
  */
-function compare_object_hash_on(callable $comparison, callable $keyFunction = null)
+function compare_object_hash_on(callable $comparison, ?callable $keyFunction = null)
 {
     $keyFunction = $keyFunction ? compose($keyFunction, 'spl_object_hash') : 'spl_object_hash';
 
diff --git a/src/Functional/CompareOn.php b/src/Functional/CompareOn.php
index b0e5cda0..2d7f2486 100644
--- a/src/Functional/CompareOn.php
+++ b/src/Functional/CompareOn.php
@@ -14,11 +14,11 @@
  * Returns a comparison function that can be used with e.g. `usort()`
  *
  * @param callable $comparison A function that compares the two values. Pick e.g. strcmp() or strnatcasecmp()
- * @param callable $reducer A function that takes an argument and returns the value that should be compared
+ * @param callable|null $reducer A function that takes an argument and returns the value that should be compared
  * @return callable
  * @no-named-arguments
  */
-function compare_on(callable $comparison, callable $reducer = null)
+function compare_on(callable $comparison, ?callable $reducer = null)
 {
     if ($reducer === null) {
         return static function ($left, $right) use ($comparison) {
diff --git a/src/Functional/Every.php b/src/Functional/Every.php
index e4d8ff62..667cd72b 100644
--- a/src/Functional/Every.php
+++ b/src/Functional/Every.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function every($collection, callable $callback = null)
+function every($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/First.php b/src/Functional/First.php
index 87db945e..20c39274 100644
--- a/src/Functional/First.php
+++ b/src/Functional/First.php
@@ -19,11 +19,11 @@
  * arguments will be element, index, collection
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return mixed
  * @no-named-arguments
  */
-function first($collection, callable $callback = null)
+function first($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Functional.php b/src/Functional/Functional.php
index 2856d6a1..5b9fc761 100644
--- a/src/Functional/Functional.php
+++ b/src/Functional/Functional.php
@@ -12,7 +12,6 @@
 
 final class Functional
 {
-
     /**
      * @see \Function\ary
      */
diff --git a/src/Functional/Group.php b/src/Functional/Group.php
index af2ed3a1..3cf054bf 100644
--- a/src/Functional/Group.php
+++ b/src/Functional/Group.php
@@ -21,7 +21,7 @@
  * @return array
  * @no-named-arguments
  */
-function group($collection, callable $callback)
+function group($collection, callable $callback): array
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
@@ -32,6 +32,11 @@ function group($collection, callable $callback)
 
         InvalidArgumentException::assertValidArrayKey($groupKey, __FUNCTION__);
 
+        // Avoid implicit precision-loss from doubles (which cannot be keys)
+        if (\is_numeric($groupKey)) {
+            $groupKey = (int) $groupKey;
+        }
+
         if (!isset($groups[$groupKey])) {
             $groups[$groupKey] = [];
         }
diff --git a/src/Functional/Head.php b/src/Functional/Head.php
index c6de5e86..3019ee86 100644
--- a/src/Functional/Head.php
+++ b/src/Functional/Head.php
@@ -17,11 +17,11 @@
  * Alias for Functional\first
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return mixed
  * @no-named-arguments
  */
-function head($collection, callable $callback = null)
+function head($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Last.php b/src/Functional/Last.php
index 8989f945..3c861d31 100644
--- a/src/Functional/Last.php
+++ b/src/Functional/Last.php
@@ -18,11 +18,11 @@
  * Callback arguments will be element, index, collection
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return mixed
  * @no-named-arguments
  */
-function last($collection, callable $callback = null)
+function last($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Memoize.php b/src/Functional/Memoize.php
index 3813e3ce..aca716b5 100644
--- a/src/Functional/Memoize.php
+++ b/src/Functional/Memoize.php
@@ -21,7 +21,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function memoize(callable $callback = null, $arguments = [], $key = null)
+function memoize(?callable $callback = null, $arguments = [], $key = null)
 {
     static $storage = [];
     if ($callback === null) {
diff --git a/src/Functional/None.php b/src/Functional/None.php
index edbd33a9..c90e4055 100644
--- a/src/Functional/None.php
+++ b/src/Functional/None.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function none($collection, callable $callback = null)
+function none($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Pick.php b/src/Functional/Pick.php
index b4be3c13..7d29f92c 100644
--- a/src/Functional/Pick.php
+++ b/src/Functional/Pick.php
@@ -24,7 +24,7 @@
  * @return mixed
  * @no-named-arguments
  */
-function pick($collection, $index, $default = null, callable $callback = null)
+function pick($collection, $index, $default = null, ?callable $callback = null)
 {
     InvalidArgumentException::assertArrayAccess($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Poll.php b/src/Functional/Poll.php
index 7c632d77..8f920d24 100644
--- a/src/Functional/Poll.php
+++ b/src/Functional/Poll.php
@@ -26,7 +26,7 @@
  * @return boolean
  * @no-named-arguments
  */
-function poll(callable $callback, $timeout, Traversable $delaySequence = null)
+function poll(callable $callback, $timeout, ?Traversable $delaySequence = null)
 {
     InvalidArgumentException::assertIntegerGreaterThanOrEqual($timeout, 0, __FUNCTION__, 2);
 
diff --git a/src/Functional/Reject.php b/src/Functional/Reject.php
index 505acc91..0efddeee 100644
--- a/src/Functional/Reject.php
+++ b/src/Functional/Reject.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function reject($collection, callable $callback = null)
+function reject($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Retry.php b/src/Functional/Retry.php
index 40ef0fa7..4466542a 100644
--- a/src/Functional/Retry.php
+++ b/src/Functional/Retry.php
@@ -29,7 +29,7 @@
  * @return mixed Return value of the function
  * @no-named-arguments
  */
-function retry(callable $callback, $retries, Traversable $delaySequence = null)
+function retry(callable $callback, $retries, ?Traversable $delaySequence = null)
 {
     InvalidArgumentException::assertIntegerGreaterThanOrEqual($retries, 1, __FUNCTION__, 2);
 
diff --git a/src/Functional/Select.php b/src/Functional/Select.php
index 93420357..eb0ab73d 100644
--- a/src/Functional/Select.php
+++ b/src/Functional/Select.php
@@ -22,7 +22,7 @@
  * @return array
  * @no-named-arguments
  */
-function select($collection, callable $callback = null)
+function select($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Sequences/ExponentialSequence.php b/src/Functional/Sequences/ExponentialSequence.php
index 0a8b8d47..b550545a 100644
--- a/src/Functional/Sequences/ExponentialSequence.php
+++ b/src/Functional/Sequences/ExponentialSequence.php
@@ -38,28 +38,28 @@ public function __construct($start, $percentage)
         $this->percentage = $percentage;
     }
 
-    public function current()
+    public function current(): int
     {
         return $this->value;
     }
 
-    public function next()
+    public function next(): void
     {
         $this->value = (int) \round(\pow($this->start * (1 + $this->percentage / 100), $this->times));
         $this->times++;
     }
 
-    public function key()
+    public function key(): ?int
     {
         return null;
     }
 
-    public function valid()
+    public function valid(): bool
     {
         return true;
     }
 
-    public function rewind()
+    public function rewind(): void
     {
         $this->times = 1;
         $this->value = $this->start;
diff --git a/src/Functional/Sequences/LinearSequence.php b/src/Functional/Sequences/LinearSequence.php
index ad99dfbb..e7345a6a 100644
--- a/src/Functional/Sequences/LinearSequence.php
+++ b/src/Functional/Sequences/LinearSequence.php
@@ -34,27 +34,27 @@ public function __construct($start, $amount)
         $this->amount = $amount;
     }
 
-    public function current()
+    public function current(): int
     {
         return $this->value;
     }
 
-    public function next()
+    public function next(): void
     {
         $this->value += $this->amount;
     }
 
-    public function key()
+    public function key(): int
     {
         return 0;
     }
 
-    public function valid()
+    public function valid(): bool
     {
         return true;
     }
 
-    public function rewind()
+    public function rewind(): void
     {
         $this->value = $this->start;
     }
diff --git a/src/Functional/Some.php b/src/Functional/Some.php
index df27e0a8..9ae81ade 100644
--- a/src/Functional/Some.php
+++ b/src/Functional/Some.php
@@ -22,7 +22,7 @@
  * @return bool
  * @no-named-arguments
  */
-function some($collection, callable $callback = null)
+function some($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Tail.php b/src/Functional/Tail.php
index 495131d2..33637ffe 100644
--- a/src/Functional/Tail.php
+++ b/src/Functional/Tail.php
@@ -18,11 +18,11 @@
  * Takes an optional callback for filtering the collection.
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @return array
  * @no-named-arguments
  */
-function tail($collection, callable $callback = null)
+function tail($collection, ?callable $callback = null)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/src/Functional/Unique.php b/src/Functional/Unique.php
index 656ac2be..cfa12442 100644
--- a/src/Functional/Unique.php
+++ b/src/Functional/Unique.php
@@ -17,12 +17,12 @@
  * Returns an array of unique elements
  *
  * @param Traversable|array $collection
- * @param callable $callback
+ * @param callable|null $callback
  * @param bool $strict
  * @return array
  * @no-named-arguments
  */
-function unique($collection, callable $callback = null, $strict = true)
+function unique($collection, ?callable $callback = null, $strict = true)
 {
     InvalidArgumentException::assertCollection($collection, __FUNCTION__, 1);
 
diff --git a/tests/Functional/ErrorToExceptionTest.php b/tests/Functional/ErrorToExceptionTest.php
index e68893e0..14c20b65 100644
--- a/tests/Functional/ErrorToExceptionTest.php
+++ b/tests/Functional/ErrorToExceptionTest.php
@@ -22,7 +22,7 @@ class ErrorToExceptionTest extends AbstractTestCase
     public function testErrorIsThrownAsException(): void
     {
         $origFn = function () {
-            \trigger_error('Some error', E_USER_ERROR);
+            \trigger_error('Some error', E_USER_DEPRECATED);
         };
 
         $fn = error_to_exception($origFn);
diff --git a/tests/Functional/GroupTest.php b/tests/Functional/GroupTest.php
index fd9fe353..62e41749 100644
--- a/tests/Functional/GroupTest.php
+++ b/tests/Functional/GroupTest.php
@@ -118,4 +118,28 @@ public function testPassNonCallable(): void
         $this->expectCallableArgumentError('Functional\group', 2);
         group($this->list, 'undefinedFunction');
     }
+
+    public function testFloatGroupKeysAreBeingCastToInteger(): void
+    {
+        $values = [5, 10, 11, 15];
+        $fn = function ($v, $k, $collection) {
+            return $v / 5;
+        };
+
+        $actual = group($values, $fn);
+        $expected = [
+            1 => [
+                0 => 5
+            ],
+            2 => [
+                1 => 10,
+                2 => 11
+            ],
+            3 => [
+                3 => 15
+            ]
+        ];
+
+        self::assertEquals($expected, $actual);
+    }
 }
diff --git a/tests/Functional/PartialMethodTest.php b/tests/Functional/PartialMethodTest.php
index 2417ed90..e52946b1 100644
--- a/tests/Functional/PartialMethodTest.php
+++ b/tests/Functional/PartialMethodTest.php
@@ -14,7 +14,6 @@
 
 class PartialMethodTest extends AbstractPartialTestCase
 {
-
     public function testWithNoArgs(): void
     {
         $method = partial_method('execute');
diff --git a/tests/Functional/RepeatTest.php b/tests/Functional/RepeatTest.php
index 9ab5e8a4..de60cb89 100644
--- a/tests/Functional/RepeatTest.php
+++ b/tests/Functional/RepeatTest.php
@@ -12,6 +12,7 @@
 
 use Functional\Exceptions\InvalidArgumentException;
 use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Runner\Version as PHPUnitVersion;
 
 use function Functional\repeat;
 
@@ -45,9 +46,23 @@ public function test(): void
     public function testNegativeRepeatedTimes(): void
     {
         $this->expectException(InvalidArgumentException::class);
-        $this->expectExceptionMessage(
-            'Functional\{closure}() expects parameter 1 to be positive integer, negative integer given'
-        );
+
+        // See https://3v4l.org/Ms79G for message formats
+        // See https://regex101.com/r/hTvW3o/1 for regex setup
+        if (\version_compare(PHPUnitVersion::id(), '9.0.0') >= 0) {
+            // PHPUnit 10 changed the wording of the exception message
+            $this->expectExceptionMessageMatches(
+                '/(Functional\\\\{closure}' // PHP < 8.4
+                . '|{closure:Functional\\\\repeat\(\):[0-9]+})' // PHP 8.4+
+                . '\(\) expects parameter 1 to be positive integer, negative integer given/'
+            );
+        } else {
+            $this->expectExceptionMessageRegExp(
+                '/(Functional\\\\{closure}' // PHP < 8.4
+                . '|{closure:Functional\\\\repeat\(\):[0-9]+})' // PHP 8.4+
+                . '\(\) expects parameter 1 to be positive integer, negative integer given/'
+            );
+        }
 
         repeat([$this->repeated, 'foo'])(-1);
     }