[Console] fix handling positional arguments

src/Symfony/Component/Console/Input/Input.php

@@ -110,6 +110,8 @@ abstract class Input implements InputInterface, StreamableInputInterface
             throw new InvalidArgumentException(sprintf('The "%s" argument does not exist.', $name));
         }
 
+        $name = \is_int($name) ? key(\array_slice($this->definition->getArguments(), $name, 1, true)) : $name;
+
         return $this->arguments[$name] ?? $this->definition->getArgument($name)->getDefault();
     }
 
@@ -122,6 +124,8 @@ abstract class Input implements InputInterface, StreamableInputInterface
             throw new InvalidArgumentException(sprintf('The "%s" argument does not exist.', $name));
         }
 
+        $name = \is_int($name) ? key(\array_slice($this->definition->getArguments(), $name, 1, true)) : $name;
+
         $this->arguments[$name] = $value;
     }
 

src/Symfony/Component/Console/Tests/Input/InputTest.php

@@ -75,6 +75,11 @@ class InputTest extends TestCase
         $input = new ArrayInput(['name' => 'foo'], new InputDefinition([new InputArgument('name'), new InputArgument('bar', InputArgument::OPTIONAL, '', 'default')]));
         $this->assertEquals('default', $input->getArgument('bar'), '->getArgument() returns the default value for optional arguments');
         $this->assertEquals(['name' => 'foo', 'bar' => 'default'], $input->getArguments(), '->getArguments() returns all argument values, even optional ones');
+
+        $input = new ArrayInput(['arg1' => 'foo'], new InputDefinition([new InputArgument('arg1'), new InputArgument('arg2')]));
+        $input->setArgument(1, 'bar');
+        $this->assertEquals('bar', $input->getArgument(1));
+        $this->assertEquals(['arg1' => 'foo', 'arg2' => 'bar'], $input->getArguments());
     }
 
     public function testSetInvalidArgument()