code: public function getBranchAlias(array $config)
      {
          if (('dev-' !== substr($config['version'], 0, 4) && '-dev' !== substr($config['version'], -4))
              || !isset($config['extra']['branch-alias'])
              || !is_array($config['extra']['branch-alias'])
          ) {
              return;
          }
  
          foreach ($config['extra']['branch-alias'] as $sourceBranch => $targetBranch) {
              // ensure it is an alias to a -dev package
              if ('-dev' !== substr($targetBranch, -4)) {
                  continue;
              }
  
              // normalize without -dev and ensure it's a numeric branch that is parseable
              $validatedTargetBranch = $this->versionParser->normalizeBranch(substr($targetBranch, 0, -4));
              if ('-dev' !== substr($validatedTargetBranch, -4)) {
                  continue;
              }
  
              // ensure that it is the current branch aliasing itself
              if (strtolower($config['version']) !== strtolower($sourceBranch)) {
                  continue;
              }
  
              // If using numeric aliases ensure the alias is a valid subversion
              if (($sourcePrefix = $this->versionParser->parseNumericAliasPrefix($sourceBranch))
                  && ($targetPrefix = $this->versionParser->parseNumericAliasPrefix($targetBranch))
                  && (stripos($targetPrefix, $sourcePrefix) !== 0)
              ) {
                  continue;
              }
  
              return $validatedTargetBranch;
          }
      }
docs: Retrieves a branch alias (dev-master => 1.0.x-dev for example) if it exists
  
  @param  array       $config the entire package config
  @return string|null normalized version of the branch alias or null if there is none