#244 - Fix MappingTransformer: add a new property to a stdClass destination - #251

Description

Fixes #244.

MappingTransformer could not add a new property to a \stdClass destination (initial_value: new \stdClass(), or keep_input on a \stdClass input): the PropertyAccessor does not consider a missing \stdClass property as writable, and only arrays had a fallback, so it threw Property '...' is not writable.

• add a \stdClass fallback, next to the array one, for a simple target property name (no ., [ or ], not empty): $result->{$targetProperty} = $transformedValue. A nested path (address.city) or an index notation ([name]) still throws, as for any other object: intermediate objects are not created;
• document it in docs/reference/transformers/mapping_transformer.md (algorithm, mapping option, note);
• add tests in MappingTransformerTest: new property with a \stdClass initial_value and with keep_input (both fail on main), and the non-simple paths that keep throwing.

PHPUnit (805 tests), PHPStan, PHP-CS-Fixer and Rector pass, and the changed files are valid PHP 8.2.

Requirements

• Documentation updates
  • Reference
  • Cookbooks
  • Changelog
• Unit tests

Breaking changes

None: this case currently throws. Nested paths and other objects behave as before.

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