From: "RubyBugs (A Nonymous)" <noreply@...>
Date: 2022-07-30T19:23:06+00:00
Subject: [ruby-core:109386] [Ruby master Bug#18950] Hash#slice fails to copy default block

Issue #18950 has been reported by RubyBugs (A Nonymous).

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Bug #18950: Hash#slice fails to copy default block
https://bugs.ruby-lang.org/issues/18950

* Author: RubyBugs (A Nonymous)
* Status: Open
* Priority: Normal
* ruby -v: 2.7.6
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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An intense weekend debugging session discovered the following root cause of a bug: `Hash#slice` returns a new Hash, which has no default block set, even if the source Hash did have a default block set.

Simplified code to reproduce:

``` ruby
# Default to an empty hash for all accessed keys
hash_with_default = Hash.new { |h, k| h[k] = {} } # => {}
hash_with_default[:a]                             # => {}
hash_with_default[:b]                             # => {}
hash_with_default                                 # => {:a=>{}, :b=>{}}

# Later, use Hash#slice
hash_sliced = hash_with_default.slice(:a, :b)     # => {:a=>{}, :b=>{}}

# Finally, access a new key
hash_sliced[:c]                                   # => nil

# Error -- that was expected to call the default block
raise "No default value" unless hash_sliced[:c] == {}
```





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