Always detect return type mismatches from docblock parents

src/Psalm/Internal/Codebase/Populator.php

@@ -361,14 +361,15 @@ class Populator
                                         $declaring_method_storage->signature_return_type
                                     )
                                 ) {
-                                    $method_storage->return_type = $declaring_method_storage->return_type;
+                                    $method_storage->return_type = clone $declaring_method_storage->return_type;
                                     $method_storage->inherited_return_type = true;
                                 } elseif (UnionTypeComparator::isSimplyContainedBy(
                                     $declaring_method_storage->return_type,
                                     $method_storage->signature_return_type
                                 )) {
-                                    $method_storage->return_type = $declaring_method_storage->return_type;
+                                    $method_storage->return_type = clone $declaring_method_storage->return_type;
                                     $method_storage->inherited_return_type = true;
+                                    $method_storage->return_type->from_docblock = false;
                                 }
                             }
                         }

src/Psalm/Internal/Provider/ReturnTypeProvider/GetClassMethodsReturnTypeProvider.php

@@ -35,5 +35,7 @@ class GetClassMethodsReturnTypeProvider implements \Psalm\Plugin\Hook\FunctionRe
         ) {
             return Type::parseString('array<string>');
         }
+
+        return null;
     }
 }

src/Psalm/Internal/Provider/ReturnTypeProvider/GetObjectVarsReturnTypeProvider.php

@@ -35,5 +35,7 @@ class GetObjectVarsReturnTypeProvider implements \Psalm\Plugin\Hook\FunctionRetu
         ) {
             return Type::parseString('array<string, mixed>');
         }
+
+        return null;
     }
 }

tests/MethodSignatureTest.php

@@ -1427,6 +1427,18 @@ class MethodSignatureTest extends TestCase
                     }',
                 'error_message' => 'ConstructorSignatureMismatch',
             ],
+            'inheritDocblockReturnFromInterface' => [
+                '<?php
+                    interface A {
+                        /** @return ?string */
+                        function foo();
+                    }
+
+                    class C implements A {
+                        public function foo() : ?string {}
+                    }',
+                'error_message' => 'InvalidReturnType',
+            ],
         ];
     }
 }