allow scalar values in return to be accepted, even when the branch has no inferred types (#5749)

src/Psalm/Internal/Analyzer/FunctionLike/ReturnTypeCollector.php

@@ -20,6 +20,8 @@ class ReturnTypeCollector
      * @param  list<Type\Union>         $yield_types
      *
      * @return list<Type\Union>    a list of return types
+     *
+     * @psalm-suppress ComplexMethod to be refactored
      */
     public static function getReturnTypes(
         Codebase $codebase,
@@ -38,6 +40,18 @@ class ReturnTypeCollector
                     $return_types[] = $stmt_type;
 
                     $yield_types = array_merge($yield_types, self::getYieldTypeFromExpression($stmt->expr, $nodes));
+                } elseif ($stmt->expr instanceof PhpParser\Node\Scalar\String_) {
+                    $return_types[] = Type::getString();
+                } elseif ($stmt->expr instanceof PhpParser\Node\Scalar\LNumber) {
+                    $return_types[] = Type::getString();
+                } elseif ($stmt->expr instanceof PhpParser\Node\Expr\ConstFetch) {
+                    if ((string)$stmt->expr->name === 'true') {
+                        $return_types[] = Type::getTrue();
+                    } elseif ((string)$stmt->expr->name === 'false') {
+                        $return_types[] = Type::getFalse();
+                    } elseif ((string)$stmt->expr->name === 'null') {
+                        $return_types[] = Type::getNull();
+                    }
                 } else {
                     $return_types[] = Type::getMixed();
                 }

tests/ReturnTypeTest.php

@@ -919,6 +919,20 @@ class ReturnTypeTest extends TestCase
                         throw new RuntimeException;
                     }
                 '
+            ],
+            'scalarLiteralsInferredAfterUndefinedClass' => [
+                '<?php
+                    /** @param object $arg */
+                    function test($arg): ?string
+                    {
+                        /** @psalm-suppress UndefinedClass */
+                        if ($arg instanceof SomeClassThatDoesNotExist) {
+                            return null;
+                        }
+
+                        return "b";
+                    }
+                '
             ]
         ];
     }