avoid by-reference warnings in filter

src/Internal/functions.php

@@ -64,13 +64,17 @@ function callback(callable $callable, int $arity = 1): \Closure
     $leadingOnly = $target->isInternal()
         || (!$target->isVariadic() && $target->getNumberOfParameters() === $arity);
 
+    // Internal functions like array_filter() pass values, not variables, so a closure taking a parameter by reference
+    // would warn on each call. Called from a wrapper, it gets the wrapper's variables instead.
+    $reflection  = new \ReflectionFunction($compiled);
+    $byReference = takesByReference($reflection);
+
     if (!$leadingOnly) {
-        return $compiled;
+        return $byReference ? static fn (...$args) => $compiled(...$args) : $compiled;
     }
 
     // A userland closure taking no more than the leading arguments already ignores the rest.
-    $reflection = new \ReflectionFunction($compiled);
-    if (!$reflection->isInternal() && !$reflection->isVariadic() && $reflection->getNumberOfParameters() <= $arity) {
+    if (!$byReference && !$reflection->isInternal() && !$reflection->isVariadic() && $reflection->getNumberOfParameters() <= $arity) {
         return $compiled;
     }
 
@@ -79,6 +83,22 @@ function callback(callable $callable, int $arity = 1): \Closure
         : static fn ($carry, $value) => $compiled($carry, $value);
 }
 
+/**
+ * Whether a function takes any of its parameters by reference.
+ *
+ * @internal
+ */
+function takesByReference(\ReflectionFunction $reflection): bool
+{
+    foreach ($reflection->getParameters() as $parameter) {
+        if ($parameter->isPassedByReference()) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 /**
  * Finds the function a callable ultimately calls: decorators are unwrapped, also when they were turned into
  * closures, and a pipeline gives its first function, which gets all of its arguments.

tests/Collections/TransformsTest.php

@@ -107,6 +107,29 @@ class TransformsTest extends \PHPUnit\Framework\TestCase
         $this->assertSeq([2], c\filter(fn ($v) => $v % 2 === 0)(self::gen([1, 2, 3])));
     }
 
+    public function testFilterWithByReferenceCallbackDoesNotWarn()
+    {
+        $warnings = [];
+        set_error_handler(function (int $level, string $message) use (&$warnings) {
+            $warnings[] = $message;
+
+            return true;
+        });
+
+        try {
+            $array = c\filter(fn (&$v) => $v > 1)([1, 2, 3]);
+            $seq   = c\filter(fn (&$v) => $v > 1)(new Seq([1, 2, 3]));
+            $both  = c\filter(fn (&$v, $k) => $k > 0)([1, 2, 3]);
+        } finally {
+            restore_error_handler();
+        }
+
+        $this->assertSame([], $warnings);
+        $this->assertSame([1 => 2, 2 => 3], $array);
+        $this->assertSeq([2, 3], $seq);
+        $this->assertSame([1 => 2, 2 => 3], $both);
+    }
+
     public function testFilterWithInternalPredicate()
     {
         $this->assertSame([1 => 'a'], c\filter(is_string(...))([1, 'a']));