As described in the YAML specification (7.1. Alias Nodes), any properties (anchor and tag) must not be specified to an alias node.
Although the library emits an error if a tag is specified to an alias node, it doesn't if an anchor is specified to an alias node.

&anchor foo: !!str *anchor # this is treated as an error.
---
&anchor foo: &anchor2 *anchor # this is not treated as an error, but it should be.

So, in this PR, the implementation has been fixed so the library would throws an parse_error if an anchor is specified to an alias node.

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.