InputBag get stub for return type (#69)

src/Stubs/5/InputBag.stubphp

@@ -0,0 +1,15 @@
+<?php
+
+namespace Symfony\Component\HttpFoundation;
+
+final class InputBag extends ParameterBag
+{
+    /**
+     * Returns a string input value by name.
+     *
+     * @template D of string|null
+     * @psalm-param D $default
+     * @psalm-return string|D
+     */
+    public function get(string $key, $default = null) {}
+}

tests/acceptance/acceptance/InputBag.feature

@@ -0,0 +1,64 @@
+@symfony-5
+Feature: InputBag get return type
+
+  Background:
+    Given I have the following config
+      """
+      <?xml version="1.0"?>
+      <psalm errorLevel="1">
+        <projectFiles>
+          <directory name="."/>
+        </projectFiles>
+
+        <plugins>
+          <pluginClass class="Psalm\SymfonyPsalmPlugin\Plugin"/>
+        </plugins>
+      </psalm>
+      """
+    And I have the following code preamble
+      """
+      <?php
+
+      use Symfony\Component\HttpFoundation\Request;
+      """
+
+  Scenario Outline: Return type is string if default argument is string.
+    Given I have the following code
+      """
+      class App
+      {
+        public function __invoke(Request $request): void
+        {
+          $string = $request-><property>->get('foo', 'bar');
+          trim($string);
+        }
+      }
+      """
+    When I run Psalm
+    Then I see no errors
+    Examples:
+      | property |
+      | query    |
+      | cookies  |
+
+  Scenario Outline: Return type is nullable if default argument is not provided.
+    Given I have the following code
+      """
+      class App
+      {
+        public function __invoke(Request $request): void
+        {
+          $nullableString = $request-><property>->get('foo');
+          trim($nullableString);
+        }
+      }
+      """
+    When I run Psalm
+    Then I see these errors
+      | Type                 | Message                                                          |
+      | PossiblyNullArgument | Argument 1 of trim cannot be null, possibly null value provided  |
+    And I see no other errors
+    Examples:
+      | property |
+      | query    |
+      | cookies  |