Rubocop brakes on 2.7 pattern matching AST nodes while tree walking, as they children may be nil

[maksar]

Following code

  case payload
  in key:
    method
  end

produces such AST (with Parser::CurrentRuby.parse, using Parser::Ruby27 parser gem version: 2.7.0.1):

s(:case_match,
  s(:send, nil, :payload),
  s(:in_pattern,
    s(:hash_pattern,
      s(:match_var, :key)), nil, # <- here
    s(:send, nil, :method)), nil)

Second children of the in_pattern s-expression is nil, which leads to

`undefined method `type' for nil:NilClass

error on line https://github.com/rubocop-hq/rubocop/blob/master/lib/rubocop/ast/traversal.rb#L57

Simplest fix (maksar@db9f1cf) for me was adding .compact, but I'm not sure it is the right thing to do.

Issue is reproducible with current master (019fe5e)

[buehmann]

Thanks! This is related to my remark here: #7605 (comment)

[iliabylich]

Why does rubocop have its own visitor implementation? parser ships with https://github.com/whitequark/parser/blob/master/lib/parser/ast/processor.rb

[buehmann]

According to Git history:

Add RuboCop::Node::Traversal (to traverse AST without mutating it)

When AST::Processor calls a hook, it takes the return value as an updated
node and tries building a modified AST. We don't want to do this.

aead344

[koic]

This issue will be solved by #7786.