diff --git a/src/Psalm/Internal/Analyzer/MethodComparator.php b/src/Psalm/Internal/Analyzer/MethodComparator.php
index 16ff2d9d98b..480d0ecedec 100644
--- a/src/Psalm/Internal/Analyzer/MethodComparator.php
+++ b/src/Psalm/Internal/Analyzer/MethodComparator.php
@@ -27,6 +27,7 @@
 use Psalm\Issue\ParamNameMismatch;
 use Psalm\Issue\TraitMethodSignatureMismatch;
 use Psalm\IssueBuffer;
+use Psalm\Storage\AttributeStorage;
 use Psalm\Storage\ClassLikeStorage;
 use Psalm\Storage\FunctionLikeParameter;
 use Psalm\Storage\MethodStorage;
@@ -35,6 +36,7 @@
 use Psalm\Type\Atomic\TTemplateParam;
 use Psalm\Type\Union;
 
+use function array_filter;
 use function in_array;
 use function strpos;
 use function strtolower;
@@ -113,6 +115,27 @@ public static function compare(
             );
         }
 
+        if (!$guide_classlike_storage->user_defined
+            && $implementer_classlike_storage->user_defined
+            && $codebase->analysis_php_version_id >= 8_01_00
+            && ($guide_method_storage->return_type
+                || $guide_method_storage->signature_return_type
+            )
+            && !$implementer_method_storage->signature_return_type
+            && !array_filter(
+                $implementer_method_storage->attributes,
+                fn (AttributeStorage $s) => $s->fq_class_name === 'ReturnTypeWillChange'
+            )
+        ) {
+            IssueBuffer::maybeAdd(
+                new MethodSignatureMismatch(
+                    'Method ' . $cased_implementer_method_id . ' is missing a return type signature!',
+                    $implementer_method_storage->location ?: $code_location
+                ),
+                $suppressed_issues + $implementer_classlike_storage->suppressed_issues
+            );
+        }
+
         if ($guide_method_storage->return_type
             && $implementer_method_storage->return_type
             && !$implementer_method_storage->inherited_return_type
@@ -862,7 +885,11 @@ private static function compareMethodSignatureReturnTypes(
                 $implementer_signature_return_type,
                 $guide_signature_return_type
             )
-            : UnionTypeComparator::isContainedByInPhp($implementer_signature_return_type, $guide_signature_return_type);
+            : (!$implementer_signature_return_type
+                && $guide_signature_return_type->isMixed()
+                ? false
+                : UnionTypeComparator::isContainedByInPhp($implementer_signature_return_type, $guide_signature_return_type)
+            );
 
         if (!$is_contained_by) {
             if ($codebase->php_major_version >= 8
diff --git a/tests/MethodSignatureTest.php b/tests/MethodSignatureTest.php
index 11f217b172f..9862b94633e 100644
--- a/tests/MethodSignatureTest.php
+++ b/tests/MethodSignatureTest.php
@@ -1569,6 +1569,37 @@ public function bar(string ...$_args): void {}
                 ',
                 'error_message' => 'MethodSignatureMismatch',
             ],
+            'noMixedTypehintInDescendant' => [
+                '<?php
+                    class a {
+                        public function test(): mixed {
+                            return 0;
+                        }
+                    }
+                    class b extends a {
+                        public function test() {
+                            return 0;
+                        }
+                    }
+                ',
+                'error_message' => 'MethodSignatureMismatch',
+                [],
+                false,
+                '8.0'
+            ],
+            'noTypehintInNativeDescendant' => [
+                '<?php
+                    class a implements JsonSerializable {
+                        public function jsonSerialize() {
+                            return 0;
+                        }
+                    }
+                ',
+                'error_message' => 'MethodSignatureMismatch',
+                [],
+                false,
+                '8.1'
+            ],
         ];
     }
 }