feature #17644 Deprecate using Form::isValid() with an unsubmitted form (Ener-Getick)

This PR was merged into the 3.2-dev branch.

Discussion
----------

Deprecate using Form::isValid() with an unsubmitted form

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #7737
| License       | MIT
| Doc PR        | not yet

> I'm calling out for you to decide how to resolve the inconsistency between the Form::isValid() method and the ``valid`` variable available in the template:
>
> - ``isValid()`` returns ``false`` if a form was not submitted. This way it is possible to write concise controller code:
>
> ```php
> $form = $this->createForm(...);
> $form->handleRequest($request);
> if ($form->isValid()) {
>     // only executed if the form is submitted AND valid
> }
> ```
>
> - ``valid`` contains ``true`` if a form was not submitted. This way it is possible to rely on this variable for error styling of a form.
>
> ```twig
> <div{% if not form.vars.valid %} class="error"{% endif %}>
> ```
>
> We have two alternatives for resolving this problem:
>
> 1. Leave the inconsistency as is.
> 2. Make ``isValid()`` return ``true`` if a form was not submitted (consistent with ``valid``)
> 3. Revert to the 2.2 behavior of throwing an exception if ``isValid()`` is called on a non-submitted form (not consistent with ``valid``).
>
> Both 2. and 3. will require additional code in the controller:
>
> ```php
> $form = $this->createForm(...);
> $form->handleRequest($request);
> if ($form->isSubmitted() && $form->isValid()) {
>     // only executed if the form is submitted AND valid
> }
> ```
>
> What do you think?

This PR implements the option 3 as it was the most chosen in #7737

Commits
-------

2c3a7cc Deprecate using Form::isValid() with an unsubmitted form

UPGRADE-3.2.md

@@ -7,6 +7,28 @@ DependencyInjection
  * Calling `get()` on a `ContainerBuilder` instance before compiling the
    container is deprecated and will throw an exception in Symfony 4.0.
 
+Form
+----
+
+ * Calling `isValid()` on a `Form` instance before submitting it
+   is deprecated and will throw an exception in Symfony 4.0.
+
+   Before:
+
+   ```php
+   if ($form->isValid()) {
+       // ...
+   }
+   ```
+
+   After:
+
+   ```php
+   if ($form->isSubmitted() && $form->isValid()) {
+       // ...
+   }
+   ```
+
 Validator
 ---------
 

UPGRADE-4.0.md

@@ -42,6 +42,25 @@ Form
  * Caching of the loaded `ChoiceListInterface` in the `LazyChoiceList` has been removed,
    it must be cached in the `ChoiceLoaderInterface` implementation instead.
 
+ * Calling `isValid()` on a `Form` instance before submitting it is not supported
+   anymore and raises an exception.
+
+   Before:
+
+   ```php
+   if ($form->isValid()) {
+       // ...
+   }
+   ```
+
+   After:
+
+   ```php
+   if ($form->isSubmitted() && $form->isValid()) {
+       // ...
+   }
+   ```
+
 FrameworkBundle
 ---------------
 

src/Symfony/Component/Form/Form.php

@@ -724,6 +724,8 @@ class Form implements \IteratorAggregate, FormInterface
     public function isValid()
     {
         if (!$this->submitted) {
+            @trigger_error('Call Form::isValid() with an unsubmitted form is deprecated since version 3.2 and will throw an exception in 4.0. Use Form::isSubmitted() before Form::isValid() instead.', E_USER_DEPRECATED);
+
             return false;
         }
 

src/Symfony/Component/Form/FormInterface.php

@@ -192,7 +192,7 @@ interface FormInterface extends \ArrayAccess, \Traversable, \Countable
     /**
      * Returns whether the form and all children are valid.
      *
-     * If the form is not submitted, this method always returns false.
+     * If the form is not submitted, this method always returns false (but will throw an exception in 4.0).
      *
      * @return bool
      */

src/Symfony/Component/Form/Tests/SimpleFormTest.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Form\Tests;
 
+use Symfony\Bridge\PhpUnit\ErrorAssert;
 use Symfony\Component\Form\Form;
 use Symfony\Component\Form\FormEvent;
 use Symfony\Component\Form\FormEvents;
@@ -315,7 +316,9 @@ class SimpleFormTest extends AbstractFormTest
 
     public function testNotValidIfNotSubmitted()
     {
-        $this->assertFalse($this->form->isValid());
+        ErrorAssert::assertDeprecationsAreTriggered(array('Call Form::isValid() with an unsubmitted form'), function () {
+            $this->assertFalse($this->form->isValid());
+        });
     }
 
     public function testNotValidIfErrors()