diff --git a/src/Composer/Command/GlobalCommand.php b/src/Composer/Command/GlobalCommand.php
index d74bf2b14..a99898e28 100644
--- a/src/Composer/Command/GlobalCommand.php
+++ b/src/Composer/Command/GlobalCommand.php
@@ -37,6 +37,14 @@ class GlobalCommand extends BaseCommand
                 return $command->isHidden() ? null : $command->getName();
             }, $application->all())));
         }
+
+        if ($application->has($commandName = $input->getArgument('command-name'))) {
+            $input = $this->prepareSubcommandInput($input, true);
+            $input = CompletionInput::fromString($input->__toString(), 3);
+            $command = $this->getApplication()->find($commandName);
+            $input->bind($command->getDefinition());
+            $command->complete($input, $suggestions);
+        }
     }
 
     /**
@@ -100,6 +108,13 @@ EOT
             return parent::run($input, $output);
         }
 
+        $input = $this->prepareSubcommandInput($input);
+
+        return $this->getApplication()->run($input, $output);
+    }
+
+    private function prepareSubcommandInput(InputInterface $input, bool $quiet = false): StringInput
+    {
         // The COMPOSER env var should not apply to the global execution scope
         if (Platform::getEnv('COMPOSER')) {
             Platform::clearEnv('COMPOSER');
@@ -122,13 +137,15 @@ EOT
         } catch (\Exception $e) {
             throw new \RuntimeException('Could not switch to home directory "'.$home.'"', 0, $e);
         }
-        $this->getIO()->writeError('<info>Changed current directory to '.$home.'</info>');
+        if (!$quiet) {
+            $this->getIO()->writeError('<info>Changed current directory to '.$home.'</info>');
+        }
 
         // create new input without "global" command prefix
         $input = new StringInput(Preg::replace('{\bg(?:l(?:o(?:b(?:a(?:l)?)?)?)?)?\b}', '', $input->__toString(), 1));
         $this->getApplication()->resetComposer();
 
-        return $this->getApplication()->run($input, $output);
+        return $input;
     }
 
     /**