[Form] Handle false as empty value on expanded choices

src/Symfony/Component/Form/Extension/Core/Type/CheckboxType.php

@@ -33,6 +33,7 @@ public function buildForm(FormBuilderInterface $builder, array $options)
         // doing so also calls setDataLocked(true).
         $builder->setData(isset($options['data']) ? $options['data'] : false);
         $builder->addViewTransformer(new BooleanToStringTransformer($options['value']));
+        $builder->setAttribute('_false_is_empty', true); // @internal - A boolean flag to treat false as empty, see Form::isEmpty() - Do not rely on it, it will be removed in Symfony 5.1.
     }
 
     /**

src/Symfony/Component/Form/Form.php

@@ -725,7 +725,9 @@ public function isEmpty()
             // arrays, countables
             ((\is_array($this->modelData) || $this->modelData instanceof \Countable) && 0 === \count($this->modelData)) ||
             // traversables that are not countable
-            ($this->modelData instanceof \Traversable && 0 === iterator_count($this->modelData));
+            ($this->modelData instanceof \Traversable && 0 === iterator_count($this->modelData)) ||
+            // @internal - Do not rely on it, it will be removed in Symfony 5.1.
+            (false === $this->modelData && $this->config->getAttribute('_false_is_empty'));
     }
 
     /**

src/Symfony/Component/Form/Tests/Extension/Core/Type/CheckboxTypeTest.php

@@ -190,4 +190,13 @@ public function testSubmitNullUsesDefaultEmptyData($emptyData = 'empty', $expect
         $this->assertSame($expectedData, $form->getNormData());
         $this->assertSame($expectedData, $form->getData());
     }
+
+    public function testSubmitNullIsEmpty()
+    {
+        $form = $this->factory->create(static::TESTED_TYPE);
+
+        $form->submit(null);
+
+        $this->assertTrue($form->isEmpty());
+    }
 }

src/Symfony/Component/Form/Tests/Extension/Core/Type/ChoiceTypeTest.php

@@ -2046,4 +2046,45 @@ public function provideTrimCases()
             'Multiple expanded' => [true, true],
         ];
     }
+
+    /**
+     * @dataProvider expandedIsEmptyWhenNoRealChoiceIsSelectedProvider
+     */
+    public function testExpandedIsEmptyWhenNoRealChoiceIsSelected($expected, $submittedData, $multiple, $required, $placeholder)
+    {
+        $options = [
+            'expanded' => true,
+            'choices' => [
+                'foo' => 'bar',
+            ],
+            'multiple' => $multiple,
+            'required' => $required,
+        ];
+
+        if (!$multiple) {
+            $options['placeholder'] = $placeholder;
+        }
+
+        $form = $this->factory->create(static::TESTED_TYPE, null, $options);
+
+        $form->submit($submittedData);
+
+        $this->assertSame($expected, $form->isEmpty());
+    }
+
+    public function expandedIsEmptyWhenNoRealChoiceIsSelectedProvider()
+    {
+        // Some invalid cases are voluntarily not tested:
+        //   - multiple with placeholder
+        //   - required with placeholder
+        return [
+            'Nothing submitted / single / not required / without a placeholder -> should be empty' => [true, null, false, false, null],
+            'Nothing submitted / single / not required / with a placeholder -> should not be empty' => [false, null, false, false, 'ccc'], // It falls back on the placeholder
+            'Nothing submitted / single / required / without a placeholder -> should be empty' => [true, null, false, true, null],
+            'Nothing submitted / single / required / with a placeholder -> should be empty' => [true, null, false, true, 'ccc'],
+            'Nothing submitted / multiple / not required / without a placeholder -> should be empty' => [true, null, true, false, null],
+            'Nothing submitted / multiple / required / without a placeholder -> should be empty' => [true, null, true, true, null],
+            'Placeholder submitted / single / not required / with a placeholder -> should not be empty' => [false, '', false, false, 'ccc'], // The placeholder is a selected value
+        ];
+    }
 }