Wrong auto-correct with block by method name replacement

[rssdev10]

Wrong replacement of a block with 2 actual arguments by SymbolProc causes an error with wrong number of arguments

slice_when{ |x| x.nil? } => slice_when(&:nil?)
=> nil? ArgumentError (wrong number of arguments (given 1, expected 0))

---

Expected behavior

slice_when{ |x| x.nil? } is incorrectly used because of two block variable should be specified here instead of one. Good behavior is to give recommendation that other method like slice_after should be used.

Actual behavior

C: Style/SymbolProc: Pass &:nil? as an argument to slice_when instead of a block.
[1, nil, 2, 3, nil, 4, 5, 6].slice_when{|x| x.nil?}.map(&:compact)
                                       ^^^^^^^^^^^^

Auto-correct produces unworkable code:

[1, nil, 2, 3, nil, 4, 5, 6].slice_when(&:nil?).map(&:compact)
#…
#>> ArgumentError (wrong number of arguments (given 1, expected 0))

Steps to reproduce the problem

[1, nil, 2, 3, nil, 4, 5, 6].slice_when{ |x| x.nil? }.map(&:compact)

RuboCop version

rubocop -V
0.61.1 (using Parser 2.5.3.0, running on ruby 2.6.0 x86_64-darwin17)

[Drenmi]

Thank you for the report! 🙇

The root cause of this problem is that the original code does not properly reveal the yielded arguments, e.g.:

slice_when { |x, _| x.nil? }

Because the second argument is implicitly discarded, RuboCop gets the signature wrong. Unfortunately it's not realistic to keep a lookup table of the block arity of all standard library methods. Even if we did, the above would still be a false positive for user implemented methods.

What we can do is mark the auto-correct of Style/SymbolProc as unsafe, which will make it disabled by default.