Mixed contains everything (#5763)

src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php

@@ -266,6 +266,10 @@ class UnionTypeComparator
         ?Type\Union $input_type,
         Type\Union $container_type
     ): bool {
+        if ($container_type->isMixed()) {
+            return true;
+        }
+
         if (!$input_type) {
             return false;
         }

tests/MethodSignatureTest.php

@@ -937,6 +937,22 @@ class MethodSignatureTest extends TestCase
                         use MyTrait;
                     }'
             ],
+            'MixedParamInImplementation' => [
+                '<?php
+                    interface I
+                    {
+                        /**
+                         * @param mixed $a
+                         */
+                        public function a($a): void;
+                    }
+
+
+                    final class B implements I
+                    {
+                        public function a(mixed $a): void {}
+                    }'
+            ],
         ];
     }