[ruby-core:109426] [Ruby master Feature#18950] Hash#slice fails to copy default block

Issue #18950 has been updated by Eregon (Benoit Daloze).


This could lead to surprising behavior, e.g.:
```ruby
cache = Hash.new { |h,k| cache[k] = {} }
cache[:a]
sliced = cache.slice(:a)
sliced[:foo] # => actually mutates cache with this proposal
```

Yes this would not be a problem if `h[k] = {}` is used as the block, but that's not always the case.

Also if we start copying the default value/block, it seems a slippery slope e.g. to copy instance variables, class, and other state too, which all seem not desirable (and even were decided against recently).

OTOH I guess copying `compare_by_identity` does make sense.

----------------------------------------
Feature #18950: Hash#slice fails to copy default block
https://bugs.ruby-lang.org/issues/18950#change-98581

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