[ruby-core:106305] [Ruby master Feature#18366] Enumerator#return_eval

Issue #18366 has been reported by sawa (Tsuyoshi Sawada).

----------------------------------------
Feature #18366: Enumerator#return_eval
https://bugs.ruby-lang.org/issues/18366

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
----------------------------------------
Some `Enumerable` methods return one or more of the receiver's elements according to the return value of a block it takes. Often, we want such evaluated value rather than the original element.

For example, suppose we want to know the character width sufficient to fit all the strings in an array:

```ruby
a = ["Hello", "my", "name", "is", "Ruby"]
```

We either have to repeat the evaluation of the block:

```ruby
a.max_by(&:length).length # => 5
```

or create a temporal array:

```ruby
a.map(&:length).max # => 5
```

both of which seem not to be optimal.

I propose to have a method `Enumerator#return_eval` that returns the evaluated value(s) of the block:

```ruby
a.max_by.return_value(&:length) # => 5
a.min_by.return_value(&:length) # => 2
a.minmax_by.return_value(&:length) # => [2, 5]
["Ava Baker", "Benjamin Anderson", "Charlie Davidson"]
.sort_by.return_value{_1.split.reverse.join(", ")}  # => ["Baker, Ava", "Anderson, Benjamin", "Davidson, Charlie"]
```
```




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