Roundtrip not possible when using identical array twice #800

tl;dr:

require "yaml"
a = [1,2,3]
YAML.load(YAML.dump({ a: a, b: a }))
# raises 'Psych::Visitors::NoAliasRuby#visit_Psych_Nodes_Alias': Alias parsing was not enabled. To enable it, pass `aliases: true` to `Psych::load` or `Psych::safe_load`. (Psych::AliasesNotEnabled)

Long form

Hey there, today I noticed an error that surprised me quite a lot. Essentially as soon as you include the same array twice in a structure that you dump through Psych, it will not be loadable using a default YAML.load. You'd have to pass specific options to make it loadable.

The problem is that dump will generate YAML in the form:

---
:a: &1
- 1
- 2
- 3
:b: *1

This preserves the array identity for the hashes stored under a and b.

That's surprising to me. I'd have expected that if load disables certain YAML features by default, that dump would also only use those on an opt-in basis. Even worse: I don't see a setting documented that would allow me to disable the alias creation. The two fixes available to me right now:

• enable alias deserialization (which I believed to be disabled by default for security considerations; though I fail to find a source for that now)
• duplicate values in my dump input to avoid alias generation

Second solution shown in code:

YAML.load(YAML.dump({ a: a, b: a.dup }))
=> {a: [1, 2, 3], b: [1, 2, 3]}