[Form] Fix merging params & files when "multiple" is enabled

[priyadi]

Q	A
Branch?	5.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets
License	MIT

If a form field is configured with the "multiple" option, the request handler merges param values with uploaded files. However, it does not merge correctly if "multiple" is enabled.

Example:

<input type="file" name="param[]" />
<input type="hidden" name="param[]" value="data1" />
<input type="hidden" name="param[]" value="data2" />

With the example, the file field will incorrectly replace the first hidden field, when they should be merged. This commit fixes the problem.

[carsonbot]

Hey!

I see that this is your first PR. That is great! Welcome!

Symfony has a contribution guide which I suggest you to read.

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• Always add tests
• Keep backward compatibility (see https://symfony.com/bc).
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Cheers!

Carsonbot

[nicolas-grekas]

array_merge_recursive would be a fit if it didn't cast objects into arrays.
FYI I moved the util method to FormUtil + tweaked it a bit.

[nicolas-grekas]

Thank you @priyadi.