Fix solving issue when provide provides a platform package that is already installed, fixes #1171

src/Composer/Installer.php

@@ -333,8 +333,15 @@ class Installer
 
             $constraint = new VersionConstraint('=', $package->getVersion());
             $constraint->setPrettyString($package->getPrettyVersion());
+
+            if (!($package->getRepository() instanceof PlatformRepository)
+                || !($provided = $this->package->getProvides())
+                || !isset($provided[$package->getName()])
+                || !$provided[$package->getName()]->getConstraint()->matches($constraint)
+            ) {
                 $request->install($package->getName(), $constraint);
             }
+        }
 
         // if the updateWhitelist is enabled, packages not in it are also fixed
         // to the version specified in the lock, or their currently installed version