diff --git a/src/Psalm/Internal/Analyzer/MethodComparator.php b/src/Psalm/Internal/Analyzer/MethodComparator.php
index 16ff2d9d98b..c4644cd3184 100644
--- a/src/Psalm/Internal/Analyzer/MethodComparator.php
+++ b/src/Psalm/Internal/Analyzer/MethodComparator.php
@@ -113,6 +113,20 @@ public static function compare(
             );
         }
 
+        if (!$guide_classlike_storage->user_defined
+            && $implementer_classlike_storage->user_defined
+            && $codebase->analysis_php_version_id >= 8_01_00
+            && !$implementer_method_storage->signature_return_type
+        ) {
+            IssueBuffer::maybeAdd(
+                new MethodSignatureMismatch(
+                    'Method ' . $cased_implementer_method_id . ' is missing a return type signature!',
+                    $implementer_method_storage->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
@@ -516,6 +530,20 @@ private static function compareMethodParams(
                 $suppressed_issues + $implementer_classlike_storage->suppressed_issues
             );
         }
+
+        if (!$guide_classlike_storage->user_defined
+            && $implementer_classlike_storage->user_defined
+            && $codebase->analysis_php_version_id >= 8_01_00
+            && !$implementer_param->signature_type
+        ) {
+            IssueBuffer::maybeAdd(
+                new MethodSignatureMismatch(
+                    'Argument ' . ($i + 1) . ' of ' . $cased_implementer_method_id . ' is missing a type signature!',
+                    $implementer_param->location
+                ),
+                $suppressed_issues + $implementer_classlike_storage->suppressed_issues
+            );
+        }
     }
 
     /**
diff --git a/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php b/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php
index c997199d650..0a48fb08e81 100644
--- a/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php
+++ b/src/Psalm/Internal/Type/Comparator/UnionTypeComparator.php
@@ -303,7 +303,7 @@ public static function isContainedByInPhp(
         Union $container_type
     ): bool {
         if ($container_type->isMixed()) {
-            return true;
+            return !!$input_type;
         }
 
         if (!$input_type) {