Access to undeclared property _propertyValidationState on simple templates

[dktapps]

Describe the bug
I initially found this and various other bugs using PHPStan:

The vast majority of these issues are the same thing, but only this one looks actively harmful as far as I can tell (most of the others are just unused use and dead code which could be prevented from generating with the right efforts).

Bear in mind that this is just PHPStan level 1, and higher levels might find more problems.

Expected behavior
The generated code shouldn't be trying to access an undeclared property. This happens in both mutable and immutable models.

Schema

{
	"$schema": "http://json-schema.org/draft-04/schema#",
	"type": "object",
	"additionalProperties": false,
	"properties": {
		"mode": {
			"enum": [
				"blacklist",
				"whitelist",
				"allow",
				"disallow"
			],
			"default": "disallow",
			"description": "The listed plugins will either be allowed or disallowed based on this setting"
		},
		"plugins": {
			"anyOf": [
				{
					"type": "array",
					"items": {
						"type": "string"
					}
				},
				{
					"type": "null"
				}
			],
			"default": [],
			"description": "List of plugins to allow or disallow"
		}
	}
}

My generator config can be seen here: https://github.com/pmmp/DataModels/blob/3f398d966d5180d68e33c8f4054ab592094ea54c/generate-schemas.php

Version:
0.21.5

[wol-soft]

The access to the dynamic _propertyValidationState actually isn't harmful. It's an intended dynamic property used to cache validation results for composition validations (anyOf/allOf eg.) to be able to determine whether a validator must be re-evaluated on a property change or not.

[dktapps]

Well, considering that it's declared here (in some cases), it looks like a bug to me. https://github.com/wol-soft/php-json-schema-model-generator/blob/master/src/SchemaProcessor/PostProcessor/Internal/CompositionValidationPostProcessor.php#L86

[dktapps]

Worth mentioning this will start generating warnings in 8.2 due to this RFC: https://wiki.php.net/rfc/deprecate_dynamic_properties