[Fixes rubocop#44] Use parser's new emit_forward_arg by default

Bump minimum RuboCop supported as we need the following commit:
rubocop/rubocop@3aeadd921fb526

.github/workflows/rubocop.yml

@@ -25,9 +25,9 @@ jobs:
         title: [ null ]
         include:
           - { os: windows, rubocop: master, ruby: mingw }
-          - { rubocop: '0.84.0', ruby: 2.4, os: ubuntu }
-          - { rubocop: '0.84.0', ruby: head, os: ubuntu }
-          - { rubocop: '0.84.0', ruby: 2.4, os: ubuntu, coverage: true, title: 'Coverage' }
+          - { rubocop: '0.87.0', ruby: 2.4, os: ubuntu }
+          - { rubocop: '0.87.0', ruby: head, os: ubuntu }
+          - { rubocop: '0.87.0', ruby: 2.4, os: ubuntu, coverage: true, title: 'Coverage' }
           - { rubocop: master, ruby: 2.7, os: ubuntu, modern: true, title: 'Specs "modern"' }
           - { rubocop: master, ruby: 2.7, os: ubuntu, internal_investigation: true, modern: true, title: 'Coding Style' }
 
@@ -87,7 +87,7 @@ jobs:
       matrix:
         os: [ ubuntu ]
         ruby: [ 2.4, 2.7 ]
-        rubocop: [ '0.84.0', master ]
+        rubocop: [ '0.87.0', master ]
 
     steps:
       - name: checkout

CHANGELOG.md

@@ -17,6 +17,7 @@
 
 ### Changes
 
+* [#44](https://github.com/rubocop-hq/rubocop-ast/issue/44): **(Breaking)** Use `parser` flag `self.emit_forward_arg = true` by default. ([@marcandre][])
 * [#86](https://github.com/rubocop-hq/rubocop-ast/pull/86): `PairNode#delimiter` and `inverse_delimiter` now accept their argument as a named argument. ([@marcandre][])
 
 ## 0.2.0 (2020-07-19)

README.md

@@ -35,8 +35,9 @@ See the [docs site](https://docs.rubocop.org/rubocop-ast) for more details.
 
 ### Parser compatibility switches
 
-The main `RuboCop` gem uses [legacy AST output from parser](https://github.com/whitequark/parser/#usage).
-This gem is meant to be compatible with all settings. For example, to have `-> { ... }` emitted
+This gem, by default, uses most [legacy AST output from parser](https://github.com/whitequark/parser/#usage), except for `emit_forward_arg` which is set to `true`.
+
+The main `RuboCop` gem uses these defaults (and is currently only compatible with these), but this gem can be used separately from `RuboCop` and is meant to be compatible with all settings. For example, to have `-> { ... }` emitted
 as `LambdaNode` instead of `SendNode`:
 
 ```ruby

lib/rubocop/ast/builder.rb

@@ -14,6 +14,8 @@ module AST
     #   parser = Parser::Ruby25.new(builder)
     #   root_node = parser.parse(buffer)
     class Builder < Parser::Builders::Default
+      self.emit_forward_arg = true
+
       NODE_MAP = {
         and:          AndNode,
         alias:        AliasNode,

rubocop-ast.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |s|
     'bug_tracker_uri' => 'https://github.com/rubocop-hq/rubocop-ast/issues'
   }
 
-  s.add_runtime_dependency('parser', '>= 2.7.0.1')
+  s.add_runtime_dependency('parser', '>= 2.7.1.4')
 
   s.add_development_dependency('bundler', '>= 1.15.0', '< 3.0')
 

spec/spec_helper.rb

@@ -8,7 +8,10 @@
 end
 
 require 'rubocop-ast'
-RuboCop::AST::Builder.modernize if ENV['MODERNIZE']
+if ENV['MODERNIZE']
+  RuboCop::AST::Builder.modernize
+  RuboCop::AST::Builder.emit_forward_arg = false # inverse of default
+end
 
 RSpec.shared_context 'ruby 2.3', :ruby23 do
   let(:ruby_version) { 2.3 }