[ruby-core:113083] [Ruby master Bug#19569] #map on Enumerator::Lazy block arguments differ from regular #map

Issue #19569 has been reported by TomNaessens (Tom Naessens).

----------------------------------------
Bug #19569: #map on Enumerator::Lazy block arguments differ from regular #map
https://bugs.ruby-lang.org/issues/19569

* Author: TomNaessens (Tom Naessens)
* Status: Open
* Priority: Normal
* ruby -v: 3.0.5...3.2.0
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
A Ruby 2 to 3 upgrade surfaced the following issue:

Ruby 2:

```
irb(main):252:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map(&:last).to_a
=> [:a, :b, :c]
```

Ruby 3:

```
irb(main):020:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map(&:last).to_a
(irb):20:in `each': undefined method `last' for 1:Integer (NoMethodError)
```

Debugging the issue, I noticed the following:

Map with a block returns only the first argument (and seems to ignore the zipped list):

```
irb(main):024:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map { |x| x }.to_a
=> [1, 2, 3]
```

It's not actually ignored though, it's just passed as a second argument:
```
irb(main):025:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map { |*x| x }.to_a
=> [[1, :a], [2, :b], [3, :c]]
```

Thus, this looks like a workaround:

```
irb(main):026:0> [1, 2, 3].to_enum.lazy.zip([:a, :b, :c]).map { |*x| x.last }.to_a
=> [:a, :b, :c]
```

But it behaves differently when leaving the `lazy` out:
```
irb(main):027:0> [1, 2, 3].to_enum.zip([:a, :b, :c]).map { |*x| x.last }.to_a
=> [[1, :a], [2, :b], [3, :c]]
```

Not sure if I'm correct in my conclusion, but to me it looks like the argument handling of `map` and `lazy_map` differs in an unexpected manner. Is this a bug/oversight, or I am simply using the different map implementations wrong?



-- 
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