作曲家分支反对Stash

Question

我有最简单的composer.json文件，我试图插入一个驻留在Stash（而不是GitHub）的私人仓库的分支（mybranch）。

{
  "config": {
  "vendor-dir": "lib"
},
  "name": "company/framework",
  "repositories": [
    {
      "type": "vcs",
      "url": "ssh://git@stash.server.private:7999/lt/php-ga.git"
    }
  ],
  "require": {
  "company/php-ga": "dev-mybranch"
  }
}

显然，你说dev-<branch>，告诉作曲家它应该拉分支，但当我运行composer update时，我总是得到：

Your requirements could not be resolved to an installable set of packages.

Problem 1
- The requested package getbutik/php-ga dev-mybranch exists as getbutik/php-ga[dev-master] but these are rejected by your constraint.

这条消息在谈论哪些限制？