Although accepted by PyYAML, `{*x: 4}` is not valid YAML: it should be
noted `{*x : 4}`. The reason is that a colon can be part of an anchor
name. See this comment from Tina Müller for more details:
https://github.com/adrienverge/yamllint/pull/550#discussion_r1155297373
Even if it's not a problem for yamllint, let's fix our tests to include
valid YAML snippets.
According to the YAML specification [^1]:
- > It is an error for an alias node to use an anchor that does not
> previously occur in the document.
The `forbid-undeclared-aliases` option checks that aliases do have a
matching anchor declared previously in the document. Since this is
required by the YAML spec, this option is enabled by default.
- > The alias refers to the most recent preceding node having the same
> anchor.
This means that having a same anchor repeated in a document is
allowed. However users could want to avoid this, so the new option
`forbid-duplicated-anchors` allows that. It's disabled by default.
- > It is not an error to specify an anchor that is not used by any
> alias node.
This means that it's OK to declare anchors but don't have any alias
referencing them. However users could want to avoid this, so a new
option (e.g. `forbid-unused-anchors`) could be implemented in the
future. See https://github.com/adrienverge/yamllint/pull/537.
Fixes#395Closes#420
[^1]: https://yaml.org/spec/1.2.2/#71-alias-nodes
Adrien Vergé