When using theCOLLECT_DENORMALIZATION_ERRORS flag during denormalization, Symfony should collectall errors and report them together in aPartialDenormalizationException.

Here is an example with two expected errors:

finalreadonlyclass Foo{publicfunction__construct(publicstring$bar,public\DateTimeInterface$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 thecontext

$foo =$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 viadefault_context inframework.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 thecreatedAt field), Symfony re-enters this code path. If the flag was provided viadefaultContext, it is still present on re-entry. Therefore, thenot_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.