I'm not sure what to do here. We can support this use case, but it causes major performance issues if the same form type is used multiple times within a request. To explain better, let's make the following distinction:

(a) Create field by type name

->add('myField', 'foo')

(b) Create field by type instance

->add('myField', new FooType())

If the above code is nested in a collection field with 10 rows, (a) will "resolve" (initialize) FooType only once, while (b) will resolve FooType 10 times. Resolving the same type multiple times within a request as in (b) results in multiple ResolvedFormType instances for the same type name and reduces the effects of internal caching. Also, resolving is expensive.

Conclusion: Passing type instances (no matter whether like in (b) or from getParent()) is a bad practice.

What should we do? I guess the safest approach would be to solve this ticket for BC reasons, but deprecate the possibility of passing FormType instances altogether in 2.2. Instead, you should register them before the FormFactory is created, i.e.

in the DI config for Symfony projects
manually for non-Symfony projects

$formFactory = Forms::createFormFactoryBuilder()
    ->addType(new FooType())
    ->getFormFactory();

What do you think?

Uh oh!

[Form] BC break: getParent cannot return Type #5221

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