[ruby-core:98492] [Ruby master Bug#16908] Strange behaviour of Hash#shift when used with `default_proc`.

Issue #16908 has been updated by Eregon (Benoit Daloze).


In other words, the current semantics of `return hash.default(nil) if hash.empty?` feel hacky and actually harmful to me.
The user probably never expects to have the default_proc called with a `nil` key in many cases.

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Bug #16908: Strange behaviour of Hash#shift when used with `default_proc`.
https://bugs.ruby-lang.org/issues/16908#change-85768

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* ruby -v: 2.7.0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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I don't have any strong opinion about this, but I observed the following behaviour which I thought was confusing. Maybe it's okay, or maybe we should change it to be more consistent.

```
hash = Hash.new{|k,v| k[v] = 0}

hash.shift # => 0
hash.shift # => [nil, 0]
```

My feeling was, both cases should return `[nil, 0]`.




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