Strange `FrozenError` on `Psych.load`

Is it intentional for the following Ruby code to raise FrozenError ?

Psych.safe_load(<<~'end', aliases: true, freeze: true)
  ---
  foo: &foo
    <<: *foo
end

Possibilities:

• Psych should accept this YAML and properly return a frozen, recursive hash.
• Psych should raise Psych::AnchorNotDefined instead of FrozenError.
• FrozenError is the legit exception here.

I have no idea if the YAML in question is valid or not at the first place.

Start by reproducing the supplied Psych.safe_load example and determine whether the YAML is valid and how recursive anchors interact with freeze: true. Compare the observed FrozenError with the possible AnchorNotDefined and successful frozen-hash outcomes; done means the behavior is explained and, if needed, covered by a regression test.