Don’t update return types to null

src/Psalm/Checker/FunctionLikeChecker.php

@@ -1258,7 +1258,7 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
                 if ($project_checker->alter_code
                     && isset($project_checker->getIssuesToFix()['MissingClosureReturnType'])
                 ) {
-                    if ($inferred_return_type->isMixed()) {
+                    if ($inferred_return_type->isMixed() || $inferred_return_type->isNull()) {
                         return null;
                     }
 
@@ -1288,7 +1288,7 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
             if ($project_checker->alter_code
                 && isset($project_checker->getIssuesToFix()['MissingReturnType'])
             ) {
-                if ($inferred_return_type->isMixed()) {
+                if ($inferred_return_type->isMixed() || $inferred_return_type->isNull()) {
                     return null;
                 }
 
@@ -1383,6 +1383,7 @@ abstract class FunctionLikeChecker extends SourceChecker implements StatementsSo
             ) {
                 if ($project_checker->alter_code
                     && isset($project_checker->getIssuesToFix()['InvalidNullableReturnType'])
+                    && !$inferred_return_type->isNull()
                 ) {
                     $this->addOrUpdateReturnType(
                         $project_checker,

src/Psalm/Type/Atomic/TNamedObject.php

@@ -60,7 +60,7 @@ class TNamedObject extends Atomic
         $class_name = array_pop($class_parts);
 
         if ($this->value === $this_class) {
-            return $class_name;
+            return 'self';
         }
 
         if ($namespace && preg_match('/^' . preg_quote($namespace) . '\\\\' . $class_name . '$/i', $this->value)) {

tests/FileManipulationTest.php

@@ -415,6 +415,42 @@ class FileManipulationTest extends TestCase
                 ['MissingReturnType'],
                 false,
             ],
+            'addSelfReturnType' => [
+                '<?php
+                    class A {
+                        public function foo() {
+                            return $this;
+                        }
+                    }',
+                '<?php
+                    class A {
+                        public function foo(): self {
+                            return $this;
+                        }
+                    }',
+                '7.1',
+                ['MissingReturnType'],
+                false,
+            ],
+            'dontAddMissingVoidReturnType56' => [
+                '<?php
+                    /** @return void */
+                    function foo() { }
+
+                    function bar() {
+                        return foo();
+                    }',
+                '<?php
+                    /** @return void */
+                    function foo() { }
+
+                    function bar() {
+                        return foo();
+                    }',
+                '5.6',
+                ['MissingReturnType'],
+                true,
+            ],
             'fixInvalidIntReturnType56' => [
                 '<?php
                     /**