When using the COLLECT_DENORMALIZATION_ERRORS flag during denormalization, Symfony should collect all errors and report them together in a PartialDenormalizationException.

Here is an example with two expected errors:

final readonly class Foo
{
    public function __construct(
        public string $bar,
        public \DateTimeInterface $createdAt,
    ) {}
}

$foo = $this->denormalizer->denormalize(
    data: ['createdAt' => ''],
    type: Foo::class,
);

Expected errors

1. Failed to create object because the class misses the "bar" property.
2. The data is either not a string, an empty string, or null; you should pass a string that can be parsed with the passed format or a valid DateTime string.

When the flag is passed via the context

$foo = $this->denormalizer->denormalize(
    data: ['createdAt' => ''],
    type: Foo::class,
    context: [
        DenormalizerInterface::COLLECT_DENORMALIZATION_ERRORS => true,
    ],
);

Both errors are correctly collected and returned.

When the flag is set via default_context in framework.yaml:

serializer:
    default_context:
        collect_denormalization_errors: true

Only one error is returned:
The data is either not a string, an empty string, or null; you should pass a string that can be parsed with the passed format or a valid DateTime string.

#Root Cause
The issue originates in the \src\Symfony\Component\Serializer\Serializer.php,
function normalize :

if (isset($context[DenormalizerInterface::COLLECT_DENORMALIZATION_ERRORS]) || isset($this->defaultContext[DenormalizerInterface::COLLECT_DENORMALIZATION_ERRORS])) {
    unset($context[DenormalizerInterface::COLLECT_DENORMALIZATION_ERRORS]);
    $context['not_normalizable_value_exceptions'] = [];
    $errors = &$context['not_normalizable_value_exceptions'];
    $denormalized = $normalizer->denormalize($data, $type, $format, $context);
}

The first time this block is hit, it checks for the flag either in $context or $defaultContext. If found, it initializes the error array with:

    $context['not_normalizable_value_exceptions'] = [];

However, during nested denormalization (e.g., when parsing the createdAt field), Symfony re-enters this code path. If the flag was provided via defaultContext, it is still present on re-entry. Therefore, the not_normalizable_value_exceptions array is reset again, losing the previously collected errors.

#My Fix

The fix is to enhance the condition with an additional check to ensure the array of errors is not already initialized:

if (
    (isset($context[DenormalizerInterface::COLLECT_DENORMALIZATION_ERRORS]) || isset($this->defaultContext[DenormalizerInterface::COLLECT_DENORMALIZATION_ERRORS]))
    && !isset($context['not_normalizable_value_exceptions'])
)

This ensures the array is only initialized once, preserving previously collected errors in recursive calls, regardless of whether the flag was passed via context or default_context.