[ruby-core:91670] [Ruby trunk Bug#15620] Block argument usage affects lambda semantic

Issue #15620 has been updated by marcandre (Marc-Andre Lafortune).

Assignee set to matz (Yukihiro Matsumoto)

Current behavior is clearly a bug due to the block passing implementation.

It's actually not clear to me why `lambda(&proc{}).lambda?` returns `false`. It doesn't seem useful and is counter-intuitive to me.

If others think we could revisit this, it has been clearly documented as such for over 10 years (r14713), so changing this behavior would be a breaking change. I would guess with very little impact. A quick search in the top 500 gems revealed a single use of `lambda(&...)` which doesn't look incompatible: https://github.com/CocoaPods/CocoaPods/blob/master/lib/cocoapods/resolver.rb#L435

I found two other uses in specs, rubocop: spec/rubocop/cop/style/stabby_lambda_parentheses_spec.rb and in vcr: spec/lib/vcr/structs_spec.rb. Neither seem problematic.

We could deprecate this use with warning and then change it?

Otherwise we could simply fix the regression.

----------------------------------------
Bug #15620: Block argument usage affects lambda semantic
https://bugs.ruby-lang.org/issues/15620#change-76932

* Author: alanwu (Alan Wu)
* Status: Open
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
* ruby -v: 
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
The following snippet demonstrate the issue:

``` ruby
def pass_after_use(&block)
  raise unless block
  lambda(&block).call
end

def direct_pass(&block)
  lambda(&block).call
end

pass_after_use do |_arg|
  puts "fine, because block is materialized into a Proc before it is passed to #lambda"
end

direct_pass do |_arg|
  puts "Raises because all args are required. This is not printed"
end

```

Output:
```
fine, because block is materialized into a Proc before it is passed to #lambda
Traceback (most recent call last):
	2: from lambda-block-pass.rb:14:in `<main>'
	1: from lambda-block-pass.rb:7:in `direct_pass'
lambda-block-pass.rb:14:in `block in <main>': wrong number of arguments (given 0, expected 1) (ArgumentError)
```

I think having the line `raise unless block` affect `Kenrel#lambda`'s  semantic is pretty surprising. Note that if I do `raise unless block_given?`, call to the lambda without arg also raises.

If I was to decide, I would always have the resulting lambda have required arguments even after multiple levels of block pass. That is, as long as the original block is a literal block.

This is either a breaking change or a regression from 2.4. The same script executes without raising in 2.4.5 (block arguments are always materialized).





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