[ruby-core:109387] [Ruby master Feature#18950] Hash#slice fails to copy default block

Issue #18950 has been updated by jeremyevans0 (Jeremy Evans).

Backport deleted (2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN)
ruby -v deleted (2.7.6)
Tracker changed from Bug to Feature

Hash#slice documentation states it returns a new hash object, not a modified copy of the receiver.  A new hash object by default does not have a default value or block. Hash#slice doesn't copy the default value either: `Hash.new(0).slice(:a)[1] # => nil`.

I don't think the current behavior is a bug, as there should be no expectation of the default value and block being copied.  Hash#except, #select, #transform_keys, #transform_values, and maybe other methods that return new hashes and not modified copies do not copy over the default value or block.

That being said, this seems like a reasonable feature request.

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Feature #18950: Hash#slice fails to copy default block
https://bugs.ruby-lang.org/issues/18950#change-98536

* Author: RubyBugs (A Nonymous)
* Status: Open
* Priority: Normal
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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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