Fix parsing of class string of unions

src/Psalm/Internal/Type/TypeParser.php

@@ -711,17 +711,16 @@ private static function getTypeFromGenericTree(
                 );
             }
 
-            $param_union_types = array_values($generic_params[0]->getAtomicTypes());
-
-            if (count($param_union_types) > 1) {
-                throw new TypeParseTreeException('Union types are not allowed in class string param');
-            }
+            $types = [];
+            foreach ($generic_params[0]->getAtomicTypes() as $type) {
+                if (!$type instanceof TNamedObject) {
+                    throw new TypeParseTreeException('Class string param should be a named object');
+                }
 
-            if (!$param_union_types[0] instanceof TNamedObject) {
-                throw new TypeParseTreeException('Class string param should be a named object');
+                $types []= new TClassString($type->value, $type, false, false, false, $from_docblock);
             }
 
-            return new TClassString($class_name, $param_union_types[0], false, false, false, $from_docblock);
+            return new Union($types);
         }
 
         if ($generic_type_value === 'class-string-map') {

tests/ClassLikeStringTest.php

@@ -859,6 +859,26 @@ function foo(A $a): void {
                         if (get_class($a) === A::class) {}
                     }',
             ],
+            'classStringUnion' => [
+                'code' => '<?php
+                    class Foo
+                    {
+                        /** @var class-string<TypeOne>|class-string<TypeTwo> */
+                        public ?string $bar = null;
+                        /** @var class-string<TypeOne|TypeTwo> */
+                        public ?string $baz = null;
+                    }
+
+                    class TypeOne {}
+
+                    class TypeTwo {}
+
+                    $foo = new Foo;
+                    $foo->bar = TypeOne::class;
+                    $foo->bar = TypeOne::class;
+                    $foo->baz = TypeTwo::class;
+                    $foo->baz = TypeTwo::class;'
+            ]
         ];
     }