TYPO3 Core

The DataMapper::getPlainValue expects a object to be a instance of TypeInterface in order to return a plain value.

The TypoInterface holds the `__toString` method - making it `a instance of Stringable`.

Objects/value objects coming from own extensions, not implementing the TypeInterface but being a `Stringable` (either by `implements \Stringable` or having the `__toString` method) throws the exception

`throw new UnexpectedTypeException('An object of class "' . get_class($input) . '" could not be converted to a plain value.', 1274799934);`

Suggestion here, being that the condition also allows a object being a stringable

From:
`if (TypeHandlingUtility::isCoreType($input)) {`

To:
`if (TypeHandlingUtility::isCoreType($input) || ($input instanceof \Stringable)) {`