Fix composer show -a usage, fixes #9304

src/Composer/Command/ShowCommand.php

@@ -584,7 +584,11 @@ EOT
 
         $matchedPackage = null;
         $versions = array();
-        $pool = $repositorySet->createPoolWithAllPackages();
+        if (PlatformRepository::isPlatformPackage($name)) {
+            $pool = $repositorySet->createPoolWithAllPackages();
+        } else {
+            $pool = $repositorySet->createPoolForPackage($name);
+        }
         $matches = $pool->whatProvides($name, $constraint);
         foreach ($matches as $index => $package) {
             // select an exact match if it is in the installed repo and no specific version was required

src/Composer/Repository/RepositorySet.php

@@ -280,6 +280,10 @@ class RepositorySet
         $request = new Request($lockedRepo);
 
         foreach ($packageNames as $packageName) {
+            if (PlatformRepository::isPlatformPackage($packageName)) {
+                throw new \LogicException('createPoolForPackage(s) can not be used for platform packages, as they are never loaded by the PoolBuilder which expects them to be fixed. Use createPoolWithAllPackages or pass in a proper request with the platform packages you need fixed in it.');
+            }
+
             $request->requireName($packageName);
         }